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
With `showLabel` being boolean, can't you just check for `value`? Or check for `=== true`, if it really needs to be this specific?