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
`_getCreateOptions` calls `_isDisabled`, neither checks the default value of the option. Might as well remove the override and leave the default from `widget.js`. Right?