sip_im_session: minor fixes

darcs-hash:20090126130503-b2873-d82bc728aed880ff8be000ca700d21903b6497ea
Invitation_state event can now be called more than once for the same state, added warning to sip_audio_session.py to see when this occurs

darcs-hash:20090126130809-58103-3b396b8e171149621efc31e88e83fa72c2cd3160