Add method to get column names in CRUD Model

I couldn't find any documentation for C5 that allows one to easily get the column names from the DB, so I just added one to your CRUD model.

I found it useful to create dynamic data import methods, so I thought I would share. For example:

```php
$colNames = $model->getColumnNames();

while(($row = fgetcsv($handle)) !== FALSE) {
    for($i=0; $i<count($colNames); $i++) {
        $dataArr[$colNames[$i]] = $row;
    }

    $allData[] = $dataArr;
}

$model->save($allData);
```

If this is already existing functionality, let me know, and I'll refactor my code. :D