Add database user.

        :param permissions: A ``(readFrom, writeTo)`` tuple

def add_database_user(self, new_username, new_password, permissions=None):
        """Add database user.

        :param permissions: A ``(readFrom, writeTo)`` tuple
        """
        url = "db/{0}/users".format(self._database)

        data = {
            'name': new_username,
            'password': new_password
        }

        if permissions:
            try:
                data['readFrom'], data['writeTo'] = permissions
            except (ValueError, TypeError):
                raise TypeError(
                    "'permissions' must be (readFrom, writeTo) tuple"
                )

        self.request(
            url=url,
            method='POST',
            data=data,
            expected_response_code=200
        )

        return True