[DOM L3 Events] initMouseEvent breaking change from D2E

Spec change: remove "buttonsArg" parameter from initMouseEvent

Why:
"buttonsArg" should be removed from the DOM L3 Events "initMouseEvent" API because this breaks the parameter order from the DOM L2 Events spec [1], as well as existing browser's implementations of said init method, for example Firefox [2]

[1] http://www.w3.org/TR/2000/REC-DOM-Level-2-Events-20001113/events.html#Events-MouseEvent
[2] https://developer.mozilla.org/en/DOM/event.initMouseEvent

End user code will be depending on the old event parameter positions, and I'd rather not be able to initialize this parameter than have it break compat. Alternately, we might consider moving it to the end of the parameter list :)

-Travis