Adapted Registration class to the new Credentials class

darcs-hash:20090518140412-58103-ab690846e19f7dc1c5ef44cbe1ad6d7f910cd3b0
Adapted Message class to the new Credentials class

darcs-hash:20090518140503-58103-c0e7fed9729cad7da1b7cd3eb66a7886d0f12f51