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
Why is this called from here? Looks to me like this should be moved to `_create` instead. That way it still runs on init.  `_setOption` and `refresh` call it directly anyway.