Simplified SDP getting/setting on Invitation class and disallow no SDP on outgoing INVITE

darcs-hash:20090126171039-58103-53d5124b66bb2aaa350ba7c5442924718240728f
Renamed set_state_*() methods of Invitation class

darcs-hash:20090127092007-58103-1964c68e12dcd5b547b895c93064a10b74e10b4d