bug fix to assigning key bindings; add an keybinding example

This:

 * fixes one bug
 * adds an example that changes keybindings

The issue was that doing something like:

    editor.keyBinding.keyboardHandler = new KeyboardHandler.named('vim');

would always fail, because the proxy was created asynchronously. Clients can work around it by creating a key handler, waiting on `onLoad`, and then setting it on the editor. This pushed that work down into the API however.

Let me know if you do want the example; I can remove if from the pull request if not.