Readonly fields

This pull requests adds `readonly_fields` to the model view. In the example below, `readonly_fields = ('date_updated', 'date_created')`
![Screen Shot 2013-03-20 at 12 51 17 AM](https://f.cloud.github.com/assets/1718372/279642/00128650-9133-11e2-89a1-7a03c5fea3bf.png)