move common code from sip_im_session to pypjua.clients.im, so that sip_send_file can import it

darcs-hash:20081128164227-b2873-06e7f5c5de886331760b04d3e2f815b3224738cf
sip_send_file.py: report nicer errors if target uri or filename is missing

darcs-hash:20081128164719-b2873-cda9ee977050579c30c7ad76e6356731d78e8df4