Added field Element divElement to class AceEditor.  Changed startEditor(...

A few changes:

1. `AceEditor` implements 2 interfaces used by  `com.google.gwt.user.client.ui.TextBox`.
  * `com.google.gwt.user.client.ui.HasText`
  * `com.google.gwt.user.client.TakesValue<String>`
2. I've changed the `startEditor()` method to use to a local copy of the div `Element` containing the editor javascript object.  If the DOM tree is large, there is a considerable performance improvement.