AUI-1364 - Make aui-toggler accessible

Hi @Robert-Frampton,

Attached is an update for https://issues.liferay.com/browse/AUI-1364.

I abstracted the accessibility logic into its' own file and wrote a unit test for it.  I did my best to use the aui-aria plugin to handle the aria attributes,  but ended up not using it.  The expand/collapse animation was throwing it off because the class attribute isn't updated until the animation is complete, and screen readers will read the 'aria-pressed' status immediately after it is clicked/entered.

Please let me know if there are any issues.

Thanks!