Add migrations and models for all database tables

To understand the migrations, [this is the database spec I followed](https://github.com/rawatson/namepending/wiki/Database-structure). All changes should reflect this spec.

There are three steps to approving this migration.

1. Ensure [the resources we wish to support in our API](https://github.com/rawatson/namepending/wiki/Resources) are solid.
2. Check if the database structure satisfies the needs of the resources, to the best of your knowledge.
3. Ensure the migrations do what the database structure asserts.