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
there are tests for this specific case as well https://github.com/jquery/jquery-ui/blob/button-icon-span/tests/unit/button/button_options.js#L28