public static HttpWebResponse create_instance(string block_id,
			VisualCaptchaSettings settings, string api_url)
        {
            string url = api_url + "/block/" + block_id + "/visual";
            return call(url, "POST", settings.get_dict());
        }
        public static HttpWebResponse create_captcha(
			Dictionary<string, string> credentials, 
			VisualCaptchaSettings settings, string ipaddr, 
			string user_agent, string api_url)
        {
            Dictionary<string, string> parameters =
                new Dictionary<string, string> (settings.get_dict ());
            foreach (string key in credentials.Keys) {
                parameters.Add (key, credentials[key]);
            }
            parameters.Add ("ip_addr", ipaddr);
            parameters.Add ("user_agent", user_agent);

            string url = api_url + "/captcha";
            return call (url, "POST", parameters);
        }