User authentication

Add ability for users to log in and log out. Resolves #10 
Would overwriting the `User.find_by_remember_token` method like I had earlier this morning be worth it? I took it out since it seemed redundant, but it was working by just passing in `cookies[:remember_token]`. I think it'd have to be a class method, but then it should be fine to change `digest` to an instance method and more of the logic related to the user would be within the `User` model.