All around accessibility

[woodybury]

Thanks for all your nice work. I'd like to add a no-brainer fix. audiojs is completely inaccessible, even the play button is not a button.

I've added the proper ARIA markup to make the UI tabbable for keyboard users and labels that provide context for screen reader users.

I've also added listeners to allow keyboard users to play/pause with space or enter, as well as fast forward/rewind the scrubber with arrow keys.