Adding second spec step for user sign up

Filling out the sign in form, I needed to see what the page looked
like, in particular what the DOM elements were.

So I fired up the `rails server` (`rails s` is the synonym) and
visited http://localhost:3000/users/sign_in. With my developer tools
(Firebug or Developer Tools) I inspected the DOM and found the
`form.edit_user` CSS selector to be reasonable.

More information about targeting elements via Capybara:

https://github.com/jnicklas/capybara#scoping