Database migrations and undo contracts deletion

![screen shot 2014-09-07 at 8 49 13 am](https://cloud.githubusercontent.com/assets/45482/4178311/697aac7a-368d-11e4-8917-a17aaa23add3.png)

Adding undo contract deletion to the GUI.

There is now a database schema migration script called upgrade_db.sh, which simply executes all the migration scripts in the db/migrations folder. I wanted to use Caribou for DB migrations, but it doesn't support encrypted databases so this will have to tide us over until I can submit a patch there.

You will need to execute this from the home directory of OpenBazaar to upgrade your DB:
```
sudo ./upgrade_db.sh <name_of_your_db>
```

For production the name of your db will be db/ob.db.