Modified Session to send a 100 reply for re-INVITES which add streams

Ignore-this: f6b9bdd6730b08c1a4947eccf8d10baa

darcs-hash:20091201155848-1e3d8-4ed7cfa3d5328002f8eccd51b69b69ceafa1ad13
Fixed ringing for incoming file transfers in sip_session script

Ignore-this: 1c70a18afe61157725d736841c5994a6

darcs-hash:20091201163843-1e3d8-29768b9be0a0505117b10f3f3f66433daaf60e1c