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
Did we discuss backwards compatibility for these? It should be possible to provide an optional enhancement that maps the old options to the new. Using `$.ui.backCompat`, if we still do that.