Perceived enhancements to the sign up feature specs

Hello,

I've made these changes to the sign up spec, which I perceive as enhancements. I am mainly measuring the enhancements through use of:
```
rspec --format documentation --order defined
```

Originally, that looked something like:
```
Sign up
  When signing up
    Should flash success and log in
```

After the changes I did, it looks like this:
```
Sign up
  The sign up page
    has a form with email address, password, and password confirmation fields
    has a link to the sign in page, for people who already have an account
  When the user signs up successfully
    they end up on the home page of the site
    they get signed in, so a sign out link is present
    a confirmation notice is shown
  Failure cases
    When required fields are not filled in
      Missing email address
        the user is notified that the email address is required
```
Please let me know what you think!