Adapted the Session 2 class to the new Credentials class

darcs-hash:20090518140822-58103-b14c638c8d8252c8e0c7bc56394a38e0b27a5c5b
Adapted sip_auto_publish_presence.py to the new Credentials class

darcs-hash:20090518140921-58103-a8e3d8097224770ed57c07d3564f07696ec094d9