renamed sip_msrp_client.py to sip_im_session.py

darcs-hash:20081125114851-b2873-85be84b435b93f3bd105d3aea5adc71cbf73a905
renamed sip_msrp_im_session to sip_im_session in docs

darcs-hash:20081125115305-b2873-64ab9d002c38745c3ca49c2320526b710ab678de