/**
  * Returns singletonian instance of this object
  *
  * @return Ai1ec_Oauth_Controller Singletonian instance of self
  */
 public static function get_instance()
 {
     if (!self::$_instance instanceof self) {
         global $wpdb;
         self::$_instance = new self($wpdb, Ai1ec_Settings::get_instance());
     }
     return self::$_instance;
 }