Models in separate files

I hope you don't mind @nathansamson, but I took the liberty of merging your [refactor branch](https://github.com/nathansamson/deis/tree/nathan/refactor) (you still show up on git blame, lol!), making sure it was [up to date](https://github.com/opdemand/deis/commit/f804ed8323ba99a2d9d48f648089ff2c42c2ea3b#diff-9dba586f38463d27c2d7fdadad3eb040) with the latest changes to `models.py` and running the tests.

I'm starting to look at moving the domain logic out of the views and this PR will help make the models easier to refactor.