'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.'

def add_user_grant(self, permission, user_id, headers=None, display_name=None):

    policy = self.get_acl(headers=headers)
    policy.acl.add_user_grant(permission, user_id, display_name=display_name)
    self.set_acl(policy, headers=headers)
