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
Or not. I think I got it now. `_getCreateOptions` runs before `_create`, but anything explicitly set by the user will still override that.