Add Rewriter for SHOW VARIABLES

[mattbucci]

        // SHOW VARIABLES LIKE emulation for sql_mode
        // Used by nextgen-gallery plugin
        if(0 === strpos($sql, "SHOW VARIABLES LIKE 'sql_mode'")) {
            // Act like MySQL default configuration, where sql_mode is ""
            $sql = 'SELECT \'sql_mode\' AS "Variable_name", \'\' AS "Value";';
        }