public function url($email, $size = null, $secure = true)
    {
        if (! is_null($size)) {
            $this->validateSize($size);
        }

        $url = $secure ? $this->httpsUrl : $this->httpUrl;
        $url .= hash('md5', strtolower(trim($email)));

        $params = array();
        $params['s'] = $size ?: $this->size;
        $params['r'] = $this->rating;
        $params['d'] = $this->imageSet;
        $url .= '?' . http_build_query(array_filter($params));

        return $url;
    }