Added method to Invitation to update Contact URI and used it in Session

NOTE: This method should not be called often, as it allocates memory that
      is only freed when the state of the Invitation object is set to
      TERMINATED.

darcs-hash:20090603161629-58103-886162607c281925b8908203b1d055a1d8d9858d
Update todo

darcs-hash:20090603175815-58252-ccb5f1ee4f6d78974e4ebaaec87314ce5f95f44b