Adapted sip_subscribe_xcap_diff.py to the new Credentials class

darcs-hash:20090518141248-58103-14e4769c8d5c41f8a3cefc37b3ef87f760815525
Adapted MSRPChat class to new Credentials class

darcs-hash:20090519131912-58103-803df1e4f755b4e1fd2f3d4a8883f248707b313f