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 `if` is always true. The previous line would throw an error if `this.icon` were undefined, so it has to be defined at this point.