KeyboardHandler : add `DEFAULT` to `BINDINGS` and remove special casing of `null` in the get / set keyboardHandler functions

This CL boxes the `null` value of the default keyboard handler in the
KeyboardHandler.DEFAULT constant.  This eliminates special casing of
`null` in the getters / setters as we always box the value (including
null) in a KeyboardHandler object.
example/keybindings : use KeyboardHandler.DEFAULT for equality check
Merge pull request #19 from rmsmith/default_keyboard_handler

KeyboardHandler : add `DEFAULT` to `BINDINGS` and remove special casing ...