Assume IPv4 addresses for RTPTransport arguments for now

darcs-hash:20090303171907-58103-8b55091520f983a6cd75e790a04020425b61ddc6
Check for IPv4 address in PJSIPUA.detect_nat_type()

darcs-hash:20090303171936-58103-5127e7a0b5d98a071e252b14d7606d68169ea6cd