code: def add_user_grant(self, permission, user_id):
          
          acl = self.get_acl()
          acl.add_user_grant(permission, user_id)
          self.set_acl(acl)
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 GS.
  
          :type permission: string
          :param permission: The permission being granted. Should be one of:
              READ|FULL_CONTROL
              See http://code.google.com/apis/storage/docs/developer-guide.html#authorization
              for more details on permissions.
  
          :type user_id: string
          :param user_id: The canonical user id associated with the GS account to
               which you are granting the permission.