Adapted sip_message.py to the new Credentials class

darcs-hash:20090518141045-58103-87b99b5de9559d4068ec5de4df1791930929eda5
Adapted sip_session.py to the new Credentials class

darcs-hash:20090518141058-58103-46b8a7b96d9df400b6915f21c015211adc3dd749