Add a key-value store entry.

        :param key: string
        :param value: string

def add_kv_store(self, key, value):
        """Add a key-value store entry.

        :param key: string
        :param value: string
        """
        data = {
            'operation': 'STORE',
            'key': key,
            'value': value
        }
        return self.post(self.make_url("/useragent-kv"), data=to_json(data),
                         headers=self.default_headers).text