Split building pjsip extension and installation steps

darcs-hash:20090721095702-58252-e4eab45f716b348bec42b257a08a8927fb404ee4
Added NAT type detection to sip_audio_session

Ignore-this: 2df86fa51b48701ebeb1d01f54bffc16

darcs-hash:20090721100307-1e3d8-531e3a2eb604fa5bf8f0cfaff924997bf4da775f