Added ice_stun_address and ice_stun_port attributes to RTPTransport

darcs-hash:20090106161957-58103-6347f949817cb3360328f44553788a08b1bd54a5
Return None for some properties of RTPTransport when in the STUN_FAILED state as well

darcs-hash:20090106172825-58103-11dc2c67d649b0219955ce2c9ed42b3222ed3631