feat(withGoogleMap): provide HOC for initialize google.maps.Map instance

* `withGoogleMap` requires a prop: `containerElement`
* `withGoogleMap` requires a prop: `mapElement`

BREAKING CHANGE: Wrap all `react-google-maps` components inside `withGoogleMap` HOC.

Before:

```js
// v5.0.0
<GoogleMapLoader
  containerElement={
    <div
      {...this.props}
      style={{
        height: "100%",
      }}
    />
  }
  googleMapElement={
    <GoogleMap
      ref={(map) => console.log(map)}
      defaultZoom={3}
      defaultCenter={{lat: -25.363882, lng: 131.044922}}
      onClick={::this.handleMapClick}>
      {this.state.markers.map((marker, index) => {
        return (
          <Marker
            {...marker}
            onRightclick={this.handleMarkerRightclick.bind(this, index)} />
        );
      })}
    </GoogleMap>
  }
/>
// or v4.0.0
<GoogleMap containerProps={{
    ...this.props,
    style: {
      height: "100%",
    },
  }}
  ref="map"
  defaultZoom={3}
  defaultCenter={{lat: -25.363882, lng: 131.044922}}
  onClick={::this.handleMapClick}>
  {this.state.markers.map((marker, index) => {
    return (
      <Marker
        {...marker}
        onRightclick={this.handleMarkerRightclick.bind(this, index)} />
    );
  })}
</GoogleMap>
```

After:

```js
// Wrap all `react-google-maps` components with `withGoogleMap` HOC
// and name it GettingStartedGoogleMap
const GettingStartedGoogleMap = withGoogleMap(props => (
  <GoogleMap
    ref={props.onMapLoad}
    defaultZoom={3}
    defaultCenter={{ lat: -25.363882, lng: 131.044922 }}
    onClick={props.onMapClick}
  >
    {props.markers.map((marker, index) => (
      <Marker
        {...marker}
        onRightClick={() => props.onMarkerRightClick(index)}
      />
    ))}
  </GoogleMap>
));
// Then, render it:
render(
  <GettingStartedGoogleMap
    containerElement={
      <div style={{ height: `100%` }} />
    }
    mapElement={
      <div style={{ height: `100%` }} />
    }
    onMapLoad={_.noop}
    onMapClick={_.noop}
    markers={markers}
    onMarkerRightClick={_.noop}
  />,
  document.getElementById('root')
);
```

Author: tomchentw

src/lib/GoogleMapLoader.js

@@ -0,0 +1 @@
+export const MAP = `__SECRET_MAP_DO_NOT_USE_OR_YOU_WILL_BE_FIRED`;

src/lib/constants.js

@@ -1,15 +1,3 @@
-export { default as GoogleMapLoader } from "./GoogleMapLoader";
-
-export { default as GoogleMap } from "./GoogleMap";
-export { default as Circle } from "./Circle";
-export { default as DirectionsRenderer } from "./DirectionsRenderer";
-export { default as DrawingManager } from "./DrawingManager";
-export { default as InfoWindow } from "./InfoWindow";
-export { default as KmlLayer } from "./KmlLayer";
-export { default as Marker } from "./Marker";
-export { default as OverlayView } from "./OverlayView";
-export { default as Polygon } from "./Polygon";
-export { default as Polyline } from "./Polyline";
-export { default as Rectangle } from "./Rectangle";
-export { default as SearchBox } from "./SearchBox";
-export { default as HeatmapLayer } from "./HeatmapLayer";
+export {
+  default as withGoogleMap,
+} from "./withGoogleMap";

src/lib/index.js

@@ -0,0 +1,106 @@
+/* global google */
+import warning from "warning";
+
+import invariant from "invariant";
+
+import getDisplayName from "react-display-name";
+
+import {
+  default as React,
+  PropTypes,
+  Component,
+} from "react";
+
+import {
+  MAP,
+} from "./constants";
+
+export default function withGoogleMap(WrappedComponent) {
+  return class Container extends Component {
+    static displayName = `withGoogleMap(${getDisplayName(WrappedComponent)})`;
+
+    static propTypes = {
+      containerElement: PropTypes.node.isRequired,
+      mapElement: PropTypes.node.isRequired,
+    };
+
+    static childContextTypes = {
+      [MAP]: PropTypes.object,
+    };
+
+    state = {
+      map: null,
+    };
+
+    handleComponentMount = this.handleComponentMount.bind(this);
+
+    getChildContext() {
+      return {
+        [MAP]: this.state.map,
+      };
+    }
+
+    componentWillMount() {
+      const {
+        containerElement,
+        mapElement,
+      } = this.props;
+      invariant(!!containerElement && !!mapElement,
+`Required props containerElement or mapElement is missing. You need to provide both of them.
+ The \`google.maps.Map\` instance will be initialized on mapElement and it's wrapped by\
+ containerElement.\nYou need to provide both of them since Google Map requires the DOM to\
+ have height when initialized.`
+      );
+    }
+
+    handleComponentMount(node) {
+      if (this.state.map || node === null) {
+        return;
+      }
+      warning(`undefined` !== typeof google,
+`Make sure you've put a <script> tag in your <head> element to load Google Maps JavaScript API v3.
+ If you're looking for built-in support to load it for you, use the "async/ScriptjsLoader" instead.
+ See https://github.com/tomchentw/react-google-maps/pull/168`
+      );
+      // https://developers.google.com/maps/documentation/javascript/3.exp/reference#Map
+      const map = new google.maps.Map(node);
+      this.setState({ map });
+    }
+
+    render() {
+      const {
+        containerElement,
+        mapElement,
+        ...restProps,
+      } = this.props;
+
+      const {
+        map,
+      } = this.state;
+
+      if (map) {
+        return (
+          React.cloneElement(containerElement, {
+          },
+            React.cloneElement(mapElement, {
+              ref: this.handleComponentMount,
+            }),
+            (<div>
+              <WrappedComponent {...restProps} />
+            </div>)
+          )
+        );
+      } else {
+        return (
+          React.cloneElement(containerElement, {
+          },
+            React.cloneElement(mapElement, {
+              ref: this.handleComponentMount,
+            }),
+            (<div />)
+          )
+        );
+      }
+    }
+  };
+}