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
This is also ill-named in button. Methods starting with `is` generally indicated a boolean getter, but this actually reads from the DOM, then sets an option. `_readDisabled` should do.