Added remote_user_agent attribute to Invitation

Ignore-this: 5415a6872d7dfbc7d9b32d1cc5ae4786

darcs-hash:20090717112441-1e3d8-013e8ead89af48588085abe73cd1d6fba02c728a
Added dummy methods to MSRPChat in order to conform to IMediaStream

Ignore-this: b1056d5d0805360d974bbbca37526d5c

darcs-hash:20090717112458-1e3d8-ebe083a160c3b8d460224eef9afdee15c86c64ca