moved common code from session classes into MSRPSession class; prepend index of the session at the prompt

darcs-hash:20081128143644-b2873-05c0aa838d56fe1c544a70c1a8940ec57c4b4f98
sip_im_send_file.py: explicititly close MSRP connection

darcs-hash:20081128143812-b2873-8937c89e08726b60049ebb3fcb4f45a7943c9a64