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
It looks to me like this well never be false, since `_getCreateOptions` runs before `_create`, and `_getCreateOptions` calls the (very misleadingly named) `_isDisabled`, which always sets `options.disabled` to a boolean. If that's correct, this check and the else are effectively dead code. Accordingly, the `null` default value is useless as well.