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
Not touched, but may need to be fixed: Spec says default is `false`, which matches the default in `widget.js`, but this is actually set to `null`. Which one is correct? Or is this changed by `_getCreateOptions` anyway?