Add comment arg to  DBUtil::create_table().

I want to set comment to table on DBUtil::create_table() method.
Because, [mroonga wrapper mode](http://mroonga.github.com/docs/userguide/wrapper.html) needs set engine on table comment.

like this...
```sql
CREATE TABLE diaries (
  id INT PRIMARY KEY AUTO_INCREMENT,
  content VARCHAR(255),
  FULLTEXT INDEX (content)
) ENGINE = mroonga COMMENT = 'engine "innodb"' DEFAULT CHARSET utf8;
```

this is very simple fix. I added argument on DBUtil::create_table() method.