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 null is a mistake for sure that should not be a string. 
The case where this is needed is when someone does something like 
```
$( "selector" ).checkboxradio({ icon: false, label: null, disabled: false });
```
 and setting a blank label text. The call to super will set the value to null so then we must restore it to its actual value.