Have engine use python-application notification system and Singleton metaclass, moved some existing Engine methods around and renamed EngineHelper to EngineStopper

darcs-hash:20090130173235-58103-b8fba3ef944fac121142b62465f12e888590102b
Simplified methods for responding to invite or re-invite in Invitation class, removed respond_to_reinvite_provisionally() method

darcs-hash:20090203175306-58103-dfc4750f3e54067845524ff2c7c9f2a37cd4855d