/**
  * Singleton pattern :
  * Get the instance of the object if it already exists
  * or create a new one.
  *
  * @access	public
  * @uses	Mailjet::ApiOverlay::$_instance
  *
  * @return instance
  */
 public static function getInstance()
 {
     if (!self::$_instance instanceof self) {
         self::$_instance = new self();
     }
     return self::$_instance;
 }