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

@@ -20,29 +20,25 @@ 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');
+import React from 'react';
-var createReactClass = require('create-react-class');
+import AddToCalendar from 'react-add-to-calendar';
-var AddToCalendar = require('react-add-to-calendar');
 
-var Example = createReactClass({
+class Example extends React.Component {
-  displayName: 'Example',
+  static displayName = 'Example';
+  state = {
+    event: {
+      title: 'Sample Event',
+      description: 'This is the sample event provided as an example only',
+      location: 'Portland, OR',
+      startTime: '2016-09-16T20:15:00-04:00',
+      endTime: '2016-09-16T21:45:00-04:00'
+    }
+  };
 
-  getInitialState: function() {
+  render() {
-    return {
-        event: {
-            title: 'Sample Event',
-            description: 'This is the sample event provided as an example only',
-            location: 'Portland, OR',
-            startTime: '2016-09-16T20:15:00-04:00',
-            endTime: '2016-09-16T21:45:00-04:00'
-        }
-    };
-  },
-
-  render: function() {
     return <AddToCalendar event={this.state.event}/>;
-  }
+  };
-});
+}
 ```
 
 ## Configuration