/**
  * Main Zn_Framework Instance
  *
  * Ensures only one instance of Zn_Framework is loaded or can be loaded.
  *
  * @since 1.0.0
  * @static
  * @see ZN()
  * @return Zn Framework - Main instance
  */
 public static function instance()
 {
     if (is_null(self::$_instance)) {
         self::$_instance = new self();
     }
     return self::$_instance;
 }