Added DTMF sending support to sip_session script

Ignore-this: 42ad40631574e3aa81be57248dc1bf71

darcs-hash:20091204154742-1e3d8-2d109628646a4f885fc48668402fa00244f7b581
Don't bit-shift g722 audio samples

Ignore-this: cf3ae04f29ba186d0dddac81f548211b

darcs-hash:20091208132424-56b4d-abf12d6c6f999c1a4f0c053826809c3ca5e55119