Sign in with Twitter

Sign in with Twitter and store token and secret in addition to username and uid so that I can make Twitter API requests on behalf of the user.

- [x] Add User model.
- [x] Add sign in/sign out UI.
- [x] Add sign in route that redirects to Twitter authentication url.
- [x] Add callback route that stores information from Twitter and signs in the user.
- [x] Add ability to sign out.

![untitled-4](https://f.cloud.github.com/assets/623/595247/b9340f8c-caef-11e2-97ec-9e01ead71aac.jpg)