User authentication

Add ability for users to log in and log out. Resolves #10 
It looks like `new_remember_token` is only used on instances within this class.

As for the `digest` method -- I think some of the code within `sign_out` should be brought into a `destroy_remember_token` method.  I'm also trying to think of a way to do the `find_by_remember_token` within this class so all of the logic with comparing against a digest version of the cookie is isolated within this class.  Was trying to come up with a name like `authenticate`, but I'm totally blanking...