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
`this.type === "checkbox" && checked` only happens twice (in different order), might still be worth to extract into a helper method. Especially here it would help.