Implement lycanthropy

http://bugs.openmw.org/issues/71

Adds the BecomeWerewolf, UndoWerewolf, and SetWerewolfAcrobatics script methods. SetWerewolfAcrobatics is a no-op since there doesn't seem to be any definitive info on what it actually does (the Acrobatics skill, like all the other skills and attributes, are handled with the other methods). It's relevant to note that there is not a UnsetWerewolfAcrobatics or anything to explicitly undo whatever it does, and there's no instances of SetWerewolfAcrobatics being called without also BecomeWerewolf.

Another change this series makes is that NpcStats is now derived from CreatureStats (Class::Npc is modified appropriately). I've cleaned up some instances of code that assumes they're separate, but there's probably some more lingering around.