Added is_busy attribute to Session.reject() and Session.end() (486/603)

darcs-hash:20090417115012-58103-bc75fa680ca58caed55f499c1705bee8073e1400
In sip_audio_session.py, reject calls with 486 if we are in a call

darcs-hash:20090417115059-58103-76601a62086d0fc1687b6c4d6ebadd1d1327c3a8