Allow passsword=None for Credentials object when no credentials are needed, take it into account in Invitation object

darcs-hash:20090127094626-58103-42d259fbce13aa8f73e5a79ff6d060fef5e46026
Added try/finally statement to SIPURI parsing function to always release memory pool

darcs-hash:20090127102952-58103-b7f7c9015a7253639fa605160e63f54aa4ada62b