Use RegisterUnitEvent to replace our home-brew event dispatcher.

Hello,

This PR gets rid of the central event dispatcher and the burden of maintaining unit-to-frames lisst by using the [frame:RegisterUnitEvent() API](http://wowpedia.org/API_Frame_RegisterUnitEvent) introduced in 5.0.4.

The code is smaller, I think it introduces less overhead by letting the dispatching happen on the C-side and it is working flawlessly since I wrote it one year ago.

Regards,
Adirelle.