sip_im_session.py: play sound on message sent/recvd

darcs-hash:20081125125812-b2873-d395a5310900403ac276c3b222df2d27b32a2652
Hacked new-style Invitation class into sip_im_session.py script with direct SDP manipulation

darcs-hash:20081125142401-58103-6894352f4407de15226b8284c7caa82141af0320
Updated OS X install documentation

darcs-hash:20081125175218-58103-2f327e2ce1a0ebad55bc57f82b49e1e591d0af9d