public function addCheck($check, $defaults = array()) {
    $this->ensureParameters(array(
      'name' => $check['name'],
      'host' => $check['host'],
      'url' => $check['url'],
    ), __METHOD__);
    $check += $defaults;
    $data = $this->request('POST', 'checks', $check);
    return sprintf('Created check %s for %s at http://%s%s', $data->check->id, $check['name'], $check['host'], $check['url']);
  }