Add comment mysql keyword support

[seyfer]

Add support for COMMENT

ALTER TABLE table CHANGE id_seance id_seance INT(11) NOT NULL COMMENT 'available';

$this->change_column('table', 'id_seance', array(
            "type"    => 'int', "limit"   => '11', "null"    => TRUE, 'comment' => "available"
        ));