Give access to original event

It wasn't possible to get the original event (very useful for drag and
drop). I added it.
Fix drop event with preventDefault

If preventDefault is not triggered, we can't use drop event. I fix it
and added an over function to disable it by default.
Forgot to call dragster over event
Added drop handler to deal with multiple drops on an element. Stopped propagation on the dragster: events to allow multiple drop zones on a single page. Added function to unbind all events.