Y.Button can render as non-submit

new `Y.Button`s always render as submit buttons. That's bad. Especially for a ToggleButton inside of a form.
See [this stackoverflow thread](http://stackoverflow.com/questions/11701804/yui3-button-click-event-is-acting-like-a-submit-type-instead-of-a-button-type), where @juandopazo commented "Yeah, that's probably a bug."

From my change to HISTORY.md:
Added type ATTR to ButtonCore to enable Button nodes to be rendered with "type" attribute
The default `type` for `<button>`s `submit`, which is not always desired, especially for ToggleButton.
 * `Button` now supports `submit` (backwards-compatible default), `button` and `reset`
 * `ToggleButton` is always rendered with `type="button"`

This isn't done yet! IE 10 fails one test, and IE in compat mode fails even more.