stop the callback for button elements, too

[usmonster]

Buttons are primarily used as form elements, behaving essentially as inputs. For consistency's sake, we should treat them as such and not fire the callback when the event target is a button.

Update: Expanded reasoning here.

[usmonster]

@ccampbell I can submit a "fine-tuned stopCallback() logic" pull request to implement my suggestions from this comment, if you agree with them. Please let me know!