public function addFilter( $filter, $callback, $priority = 10 )
    {
        if ( !isset( $this->filters[ $filter ] ) )
        {
            $this->filters[ $filter ] = array();
        }

        $this->filters[ $filter ][] = array(
            'priority' => $priority,
            'callback' => $callback,
        );

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

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