Add migration styles

* Default non-presence-validated string fields to empty string to avoid nil values
* Put timestamps at the top when creating a table for consistency

Screenshot of db/schema with consistent use of timestamps at top:
![screen shot 2014-04-16 at 2 30 38 pm](https://cloud.githubusercontent.com/assets/601515/2725586/79a862ae-c5ae-11e3-9fc2-c0d4caed67db.png)

Screenshot of db/schema without consistent use of timestamps at top:

![screen shot 2014-04-16 at 2 31 03 pm](https://cloud.githubusercontent.com/assets/601515/2725593/8cff9480-c5ae-11e3-8670-d726e08ce158.png)