public function country($country = null)
  {
    if (null === $country)
    {
      return $this->responseConfig['country'];
    }

    if (false === in_array(strtolower($country), $this->possibleLocations))
    {
      throw new InvalidArgumentException(sprintf(
        "Invalid Country-Code: %s! Possible Country-Codes: %s",
        $country,
        implode(', ', $this->possibleLocations)
      ));
    }

    $this->responseConfig['country'] = strtolower($country);

    return $this;
  }