rewrite Crafty.unbind

see discussion at issue 417 --
https://github.com/craftyjs/Crafty/issues/417

This implements (and documents) the behavior suggested by @mpetrovic at https://github.com/craftyjs/Crafty/issues/417#issuecomment-13556902

I haven't had time to test this patch, I'm just posting it now to contribute to the conversation.