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
Until here the method is an exact replica from the implementation in `button.js`. I wonder if there is some way to reduce that duplication. Merging both widgets into a single component is not the way to go, as we've learned...