add a wrappedComponent ref on the Injector instance

[rossipedia]

As much as we'd like to keep things declarative in React, sometimes you have to get your hands dirty with imperative operations.

This PR allows the instance of the wrapped component to be accessed, rather than just the component class:

@inject('store')
class Child extends Component {
  ...

  doSomething() {
    ...
  }
}

/**
At this point, Child is a MobXStoreInjector wrapping the original Child,
and it has a property `wrappedComponent` which refers to the
original `Child` class itself.
*/

class Parent extends Component {
  constructor(props) {
    super(props);

    this.child = null;
  }

  onSomething = () => {
    // Currently this throws as child.wrappedComponent is undefined
    this.child.wrappedComponent.doSomething();
  }

  render() {
    const store = createStore();
    return (
      <Provider store={store}>
        <Child ref={instance => this.child = instance} />
      </Provider>
    );
  }
}

[rossipedia]

would wrappedInstance be a more appropriate property name?

[mweststrate]

Merged this manually as #90. The instance will be exposed as wrappedInstance (wrappedComponent would break the current api and has it's own uses, like accessing propTypes). Will be part of next release. Thanks for the contribution!