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 the only place where `_setClasses` is called. Might be better to inline the method here, optionally extracting other parts that are duplicated elsewhere (haven't checked). Inlining would get rid of the weak name (it does a lot more then setting classes).