public function usage($start = false, $end = false, $period = null)
    {
        $params = array();

        if ($start) {
            $params['start'] = $start;
        }
        if ($end) {
            $params['end'] = $end;
        }
        if (isset($period)) {
            $params['period'] = $period;
        }

        return $this->_user->get('account/usage', $params);
    }