Use PostgreSQL instead of MongoDB

 - Introduce config package. Contains
   types for database configuration backed
   by YAML
 - Include example yaml config file (must be
   renamed to db/database.yaml)
 - Update list of dependencies
 - Add Id field to entities.User
 - A couple of smaller refactorings
Support migrations using the Sequel gem (Ruby)

 - Include example migration for a very basic
   "users" table
Refactor routing code