AUI-1353 - Make aui-datepicker accessible

Hi Marc,

Attached is an update for https://issues.liferay.com/browse/AUI-1353.

I gave a shot at binding the event like 'keyup, focus' but ran into the same issue.  I tried a solution that explicitly calls the handler on initialization if the trigger is an input.  It seems to work well.  The only weird thing is that after the popover is closed, the tab order restarts from the top.   I'm not sure if there is a way around it.

Please let me know your thoughts.

Thanks!