Adapted Session2 to new basic SIP objects and Invitation

Ignore-this: 731232473208573317300e873218b241

darcs-hash:20090616100700-1e3d8-3bdb7f447fda94896813150baaaf1ea15cae758f
Adapted sip_audio_session to new basic SIP objects

Ignore-this: 384e31ce39f0f0e9a5b8b019365a4fbd

darcs-hash:20090616100723-1e3d8-40ad0f64c5cb5e75266a49927bf8e4f39cff83e1