Add user accounts to system

The user logic needs tests. But I'm rushing and sick.

No authorization yet (TODO with `cancan`), so anyone can do anything, but at least there's an account system.

![screen shot 2013-08-14 at 5 53 18 pm](https://f.cloud.github.com/assets/740289/965505/1bd8c436-052c-11e3-990a-ffecff6bb79a.png) . ![screen shot 2013-08-14 at 5 53 31 pm](https://f.cloud.github.com/assets/740289/965508/1fe1816c-052c-11e3-9283-4b9858625890.png)

Tests that need to be written at a later date:

- Test proper assignment of Incident to a user
- Test the existence of the anonymous warning if not logged in
- Test the `scope :by_user` on `Incident`
- Test `User#gravatar`