public function listCampaigns($limit = null, $offset = null)
    {
        $data = array();
        if (!empty($limit)) {
            $data['limit'] = $limit;
        }
        if (!empty($offset)) {
            $data['offset'] = $offset;
        }
        $requestResult = $this->sendRequest('campaigns', 'GET', $data);

        return $this->handleResult($requestResult);
    }