AUI-1353 Make aui-datepicker accessible 

Hey Mike. I couldnt get the datepicker to work with the keyboard only for the text input triggers on the demo. I started to take a stab at it, but Im not really sure where we should add the code to listen on focus for those guys. Maybe some kind of delegate listener, or maybe we .show the popover during useInputNode if we detected the node is an input type=text. Lets talk about this one when you have some time