/**
  * $options array includes login parameters of merchant and optional currency.
  *
  * @param array $options
  */
 public function __construct($options = array())
 {
     $this->required_options('login, password, signature', $options);
     if (isset($options['currency'])) {
         self::$default_currency = $options['currency'];
     }
     $this->options = $options;
 }