mouse buttons are correct in IE > 8

As I commented in issue #52, this sets the correct LMB number in the mousedown, mouseup events on IE >= 9.

(Source, for IE >= 9: http://msdn.microsoft.com/en-us/library/ie/ff974877%28v=vs.85%29.aspx)
(Source, for IE < 9: http://www.quirksmode.org/js/events_properties.html#button)