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 and following lines replicate too much code from `_enhance`. It looks similar, but isn't the same, which is probably even worse then a straight copy.