AUI- 1379 - Create an aui-node-accessible submodule

Hi @Robert-Frampton,

Attached is an update for https://issues.liferay.com/browse/AUI-1379.

As per @mairatma's recommendation, I used the ```onceAfter``` method to prevent multiple bindings, and squashed that change into df173db.  I also moved the node accessible test into a new file.

Please let me know if there are any issues.

Thanks!