Fix Using AFTER and COMMENT in correct order in DBUtil

In DBUtil class SQL queries created with method add_fields are wrong if we have defined field 'after' and 'comment'

For example this code
```
		\Fuel\Core\DBUtil::add_fields('test', array(
			'col1'	=> array('constraint' => 50, 'type' => 'varchar', 'after' => 'id', 'comment' => 'My first column')
		));
``
will throw a SQL exception.