code: def add_user_grant(self, permission, user_id, headers=None,
                         display_name=None):
          
          policy = self.get_acl()
          policy.acl.add_user_grant(permission, user_id,
                                    display_name=display_name)
          self.set_acl(policy, headers=headers)
docs: Convenience method that provides a quick way to add a canonical
          user grant to a key.  This method retrieves the current ACL,
          creates a new grant based on the parameters passed in, adds that
          grant to the ACL and then PUT's the new ACL back to S3.
  
          :type permission: string
          :param permission: The permission being granted. Should be one of:
                             (READ, WRITE, READ_ACP, WRITE_ACP, FULL_CONTROL).
  
          :type user_id: string
          :param user_id:     The canonical user id associated with the AWS
                              account your are granting the permission to.
  
          :type display_name: string
          :param display_name: An option string containing the user's
                               Display Name.  Only required on Walrus.