Added CompositionEvents to nativeEvents for providing IME's stasus.

Hi.

I added CompositionEvents to nativeEvents.

This is useful when CJK language users want to observe IME's status changes like below.

```javascript
tinymce.init({
  setup: function(editor) {
    editor.on('compositionstart', function() {
      console.log('IME session started');
    });
    editor.on('compositionend', function() {
      console.log('IME session ended');
    });
  }
});
```

To get more information about CompositionEvents, please read http://www.w3.org/TR/DOM-Level-3-Events/#event-type-compositionstart .