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
It actually should just be a return to avoid the next line where we update the where we update the associated radio buttons. However unless im mistaken this if and return are not needed at all  the disabled check in `.on` will make it so this method is never called if it is disabled so there is no way to ever have this `if` be true