Added checks for SDP media stream transport for incoming sessions

Ignore-this: f1df1220717e0624ee20bbed21384357

darcs-hash:20091223141811-1e3d8-cab01627c739fd4bfb5bc666ec1a5d364015179f
Fixed Session.end to not do anything if INVITE session was not started

Ignore-this: 2a14e0abd7e7f79f3ef31cd11a1f63ea

darcs-hash:20091223145003-1e3d8-f64c83289f9608a005126576fa49712afd0002e9