initial attempt to add piece movement

Hi fellow Jeff H,
I'm planning to use the experimental jQuery widget as part of a larger Rails web app, and made a few additions to meet my goals.  I've added draggable and clickable piece movement, plus highlighting and an option to pass a post-move function.

It seems to work well enough for my needs, but frankly I'm a pretty novice javascript coder so I'm sure there's better ways to implement this stuff.  In any case, I hope this might help push the jQuery widget toward completion.

Thanks for your great work on chess.js!