private function request($url, $method = 'GET', $postfields = array())
    {
        curl_setopt_array($this->ch, array(
            CURLOPT_USERAGENT => 'VK/1.0 (+https://github.com/vladkens/VK))',
            CURLOPT_RETURNTRANSFER => true,
            CURLOPT_SSL_VERIFYPEER => false,
            CURLOPT_POST => ($method == 'POST'),
            CURLOPT_POSTFIELDS => $postfields,
            CURLOPT_URL => $url
        ));

        return curl_exec($this->ch);
    }