enable create table in sqlite3

Modified classes/dbutil.php to enable create table in sqlite3.
Previous code can't create table with primary index in sqlite3,
because previous code generate
```sql
PRIMARY KEY 'id' ('id')
```
but sqlite3 supports
```sql
PRIMARY KEY ('id')
```

I used config/db.php setting string to check whether the database is sqlite3.

The following code using `Database_Connection` is another way to do it.
```php
$sql .= ",\n\tPRIMARY KEY ";
if (strtolower(\Config::get('db.'.($db ? $db : \Config::get('db.active')).'.type')) === 'pdo')
{
  if (strtolower(\Database_Connection::instance($db)->driver_name()) !== 'sqlite')
  {
    $sql .= $key_name;
  }
}
$sql .= $key_name." (" . implode(', ', $primary_keys) . ")";
```