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
No null means use the value from the dom, which is handled in create. We had a rather lengthy discussion about this in one of the meetings, for both this and selectmenu. The decision was that null has the special meaning of checking the dom prop value, and using that. Where as false would mean even if disabled is set in the dom it will be enabled when the widget is initialized. 

This also means that you can never actually get null as the return value of `$( selector ).button( "option", "disabled" );` it will always be true or false because it will be set from the dom.

Note: while the actual option value of null is used in selectmenu the api is currently incorrect in saying the default is false.