Update example code to use React Class syntax (#16)

* Update to use React Class syntax

This is the recommended syntax to use now, and as such will be more familiar to beginners. :)

* Converted old requires to import statements.

README.md

@@ -20,15 +20,12 @@ npm install react-add-to-calendar --save
 You’ll need to install React and Moment separately since they are not included in the package. Below is a simple example on how to use the Add to Calendar button in a React view.
 
 ```js
-var React = require('react');
-var createReactClass = require('create-react-class');
-var AddToCalendar = require('react-add-to-calendar');
+import React from 'react';
+import AddToCalendar from 'react-add-to-calendar';
 
-var Example = createReactClass({
-  displayName: 'Example',
-
-  getInitialState: function() {
-    return {
+class Example extends React.Component {
+  static displayName = 'Example';
+  state = {
     event: {
       title: 'Sample Event',
       description: 'This is the sample event provided as an example only',
@@ -37,12 +34,11 @@ var Example = createReactClass({
       endTime: '2016-09-16T21:45:00-04:00'
     }
   };
-  },
 
-  render: function() {
+  render() {
     return <AddToCalendar event={this.state.event}/>;
-  }
-});
+  };
+}
 ```
 
 ## Configuration