User authentication

Refactored user authentication code. Biggest change was creating a new `RememberToken` class so that the logic around the remember tokens wasn't spread out among `User` and `Session`. Also the first time I wrote tests for custom controller methods, so feedback on those would definitely be appreciated. Thanks to @yaychris for the refactor help and bettering my Vim-fu.
Believe I've got that [here](https://github.com/andycandrea/hnews/blob/fc8bf6c54d5ba816249fa748c0c46375d69dc214/db/migrate/20140625135342_add_remember_token_to_users.rb#L4).