`idAttribute` set to _name_ for `Backgrid.Column`

Hi @wyuenho!
I've made a small change for easy search within the columns collection.
I think it's more convinient to use
```javascript
backgridInstance.columns.get('file_name');
```
instead of 
```javascript
backgridInstance.columns.findWhere({name: 'file_name'});
```