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.
`let` lazy loads, so it doesn't create the user until the first time you reference the variable `user`. you can use `let!` to eager load it instead.