public function post(Array $postVars, Array $headers, $url=null)
    {
        try {
            $this->guzzleResponse = $this->_client->request(
                'POST',
                $url,
                array('multipart' => $postVars, 'headers' => $headers)
            );
        } catch (RequestException $e) {
            throw new GreenhouseAPIResponseException($e->getMessage(), 0, $e);
        }
        
        return (string) $this->guzzleResponse->getBody();
    }