This removes auto-registration, committed by Martin r30961.
Realize this is a contentious topic but Brecht and myself both would rather opt-in registration.

TODO:
- addons need updating.
- class list will be modified to use weakrefs (should have been done for existing system too).
- will move bpy.types.(un)register functions into bpy.utils.(un)register_class, currently including these functions in a type list is internally ugly, scripts which loop over types also need to check for these.