Button icon span

Working branch right now up for functional review for button re-write demo at http://view.jqueryui.com/button-icon-span/demos/button/

Spec: http://wiki.jqueryui.com/w/page/71756381/Button
@jzaefferer no backcompat has not been discussed at any point yet. It would likely require little as far as js however it would likely require some addition css to make multiple icons work but I have not tried yet.