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

[derek]

As issue #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), 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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