Defaulting Y.ButtonCore's template to type=button (#973, #968)

As issue https://github.com/yui/yui3/pull/968 mentions, any Buttons rendered without a `host`/`srcNode` config option will default to `type=submit`.  This is the correct behavior for any `<button>` element without a `type` attribute ([spec](http://www.whatwg.org/specs/web-apps/current-work/multipage/the-button-element.html#the-button-element)), however, in the context of Y.Button, that is certainly not the ideal default.  It is much more likely that someone would like to listen in on a click events instead of submit the parent form.  So this change updates the default `TEMPLATE` to include `type=button`.  

If the intent actually is to have `type=submit`, the implementer can either override Y.ButtonCore.prototype.TEMPLATE, or specify it in a new Y.Node instance in the config (as a new test does).

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