Bugfix for AudioTransport.get_local_media() on incoming call

darcs-hash:20090205174538-58103-d125a40d281e71d5ed2cdbc45c3c8805692c4915
Added is_held attribute to Session object to indicate if we are put on hold by the remote party

darcs-hash:20090205175135-58103-bcd5b6d264f4bbb50c9822437242a746b99a0cfc