Add Initial Models, Migrations, Fixtures, and Tests

Here's the initial implementation of the data model as defined in our google doc. I've also included a few basic tests and fixtures for us to use.

User passwords are hashed and salted automatically (both values are stored together in the `password_digest` column). The `password` and `password=` getter/setter will transparently encrypt or decrypt the password as needed.

@ryanseys 
@oharaandrew314 
I suppose in reality the default when someone signs up will be owner level... but don't you think it would be better to default to low permissions and explicitly raise them when necessary (risking someone being locked out of their account), rather than defaulting to god permissions and lowering when necessary (risking someone being able to wreck havok)?