rename msrpim.py -> msrp.py

darcs-hash:20090218065640-b2873-ad9f74108e7eb1408e5f17963eed383e57ada3be
msrp.py: rename MSRPIMSession to MSRPChat

darcs-hash:20090218071600-b2873-5b11273afecf89e475012783c09c7b1c18093015