Convenience method that provides a quick way to add a canonical
        user grant to a bucket.  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 recursive: boolean
        :param recursive: A boolean value to controls whether the command
                          will apply the grant to all keys within the bucket
                          or not.  The default value is False.  By passing a
                          True value, the call will iterate through all keys
                          in the bucket and apply the same grant to each key.
                          CAUTION: If you have a lot of keys, this could take
                          a long time!
                          
        :type display_name: string
        :param display_name: An option string containing the user's
                             Display Name.  Only required on Walrus.

def add_user_grant(self, permission, user_id, recursive=False,
                       headers=None, display_name=None):
        """
        Convenience method that provides a quick way to add a canonical
        user grant to a bucket.  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 recursive: boolean
        :param recursive: A boolean value to controls whether the command
                          will apply the grant to all keys within the bucket
                          or not.  The default value is False.  By passing a
                          True value, the call will iterate through all keys
                          in the bucket and apply the same grant to each key.
                          CAUTION: If you have a lot of keys, this could take
                          a long time!
                          
        :type display_name: string
        :param display_name: An option string containing the user's
                             Display Name.  Only required on Walrus.
        """
        if permission not in S3Permissions:
            raise self.connection.provider.storage_permissions_error(
                'Unknown Permission: %s' % permission)
        policy = self.get_acl(headers=headers)
        policy.acl.add_user_grant(permission, user_id,
                                  display_name=display_name)
        self.set_acl(policy, headers=headers)
        if recursive:
            for key in self:
                key.add_user_grant(permission, user_id, headers=headers,
                                   display_name=display_name)