Added self and threading check to detect_nat_type()

darcs-hash:20081210204243-58103-d8bc446b95c9488d43998351d2186223b970f515
Try to detect the NAT type and display the result in sip_audio_session.py

darcs-hash:20081210204304-58103-af15649f224c70f3e66c6d4b6e018d839162cf68