public function addAction( $action, $callback, $priority = 10 )
    {
        if ( !isset( $this->actions[ $action ] ) )
        {
            $this->actions[ $action ] = array();
        }

        $this->actions[ $action ][] = array(
            'priority' => $priority,
            'callback' => $callback,
        );

        if ( 1 < count( $this->actions[ $action ] ) )
        {
            usort( $this->actions[ $action ], array( $this, '_sortByPriority' ) );
        }

        if ( $this->debug )
        {
            echo PHP_EOL . '+ACTION: ' . $action . ' -> '
                    . $this->_serializeCallback( $callback )
                    . ' + Priority: ' . $priority
                    . PHP_EOL;
        }
    }