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
Can this be simplified to `if ( this.icon ) {`? I don't see why we need to explicitly check for undefined, since its expected to be a jQuery object. Anything falsy needs to be overwritten. Repeat below.