Multiple binds for same event

[hfossli]

In a OOP setup where multiple objects are tapping in to the same event they should be able to unbind without side effects.

Suggestion
I think on(...) should return a unique id (int) so that it is possible to remove the binding with off(identifier). We should also introduce a convenience method offAllForEvent(event) or something like that for those cases where you actually want to remove all.