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
The dev can also pass `null` `$( selector ).button({ disabled: null });` if this happens `options.disabled` will be set to a `bool` in `_getCreateOptions` but it will be set back to `null` by the options object passed by the dev as this overrides anything from `_getCreateOptions`