Add ability to pass custom events to drop zones for drag enter/leave.

Came across this use case for something where we have collapsed elements, and we want to be able to expand them on hover (after a certain amount of time), so we need the ability to know when something has entered/left while dragging, and trigger events for both.
If you don't supply one, the parsed value is just angular.noop, so it won't actually do anything anyways. It seems a bit easier to do it that way, rather than do any sort of existential checking (and this is similiar to how the drop handlers work up above).