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
`"null"`?? That's not supposed to be a string, is it?

Also I don't understand why this, along with the related lines below, is necessary.