Add postgres, activerecord and a Code model

This introduces:

* ActiveRecord with the postgres adaptor (and a *database.yml* file)
* A `Rakefile`, with the AR tasks for migrations
* A `Code` model and migration, which allows codes to be stored in the DB instead of in memory

I made the assumption that this was the way that the project should go - using a database for persistence. As it's been deployed to heroku, postgres seemed like the obvious choice.