User authentication

Add ability for users to log in and log out. Resolves #10 
Would you imagine the code looking something like the following?

```ruby
def self.find_by_remember_token(token)
  where(remember_token: digest(token))
end

# => User.find_by_remember_token(cookies[:remember_token])
```

If so, then `digest` would have to remain a class method.