feat(Spinner): Added Spinner element

src/index.js

@@ -1,11 +1,13 @@
 import Button from './button';
 import Dropdown from './dropdown';
 import Input from './input';
+import Spinner from './spinner';
 
 const mdlReact = {
   Button,
   Dropdown,
-  Input
+  Input,
+  Spinner
 };
 
 module.exports = mdlReact;

src/spinner.js

@@ -0,0 +1,27 @@
+import React from 'react';
+import classNames from 'classnames';
+
+class Spinner extends React.Component {
+  componentDidMount() {
+    window.componentHandler.upgradeElement(this.root);
+  }
+
+  componentWillUnmount() {
+    window.componentHandler.downgradeElements(this.root);
+  }
+
+  render() {
+    const classes = classNames(
+      'mdl-spinner mdl-js-spinner is-active',
+      this.props.className
+    );
+
+    return (<div ref={ (node) => { this.root = node; } } className={ classes } />);
+  }
+}
+
+Spinner.propTypes = {
+  className: React.PropTypes.string
+};
+
+export default Spinner;