Users can create accounts, log in to their accounts, OR log in as a guest

"Logging in as a guest" adds a <code>:username</code> to the session cookies. It also defines the existence of this cookie as being <code>signed_in?</code> in <code>application_controller</code>.

I also move all the signing in and creating accounts on 1 page, the <code>session#new</code> page.
I like the explicit nature of  'session[:username] = nil`.