Adding first spec step for viewing singup page

By using `rake routes` I was able to inspect what the existing routes
were for my Rails application. It looks like '/users/sign_in' is the
path to the Devise signup page.

Once the change was made, I ran `rspec` and an
ActiveRecord::StatementInvalid exception was raised (see below).

Rspec said that it could not find the 'users' table. This is because
our *test* database has not been created. When `rake db:migrate` is
run, only the *development* database was modified.

So I ran `rake db:test:prepare` and then ran `rspec`.

```
The following failure occurred when I ra
Failures:

  1) user signup attempting to login without an account
     Failure/Error: visit('/users/sign_in')
     ActiveRecord::StatementInvalid:
       Could not find table 'users'

Finished in 0.04759 seconds
2 examples, 1 failure, 1 pending

Failed examples:

rspec ./spec/features/user_signup_spec.rb:4 # user signup attempting…
```
Adding seond 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