Simplify PagesController#index action and specs

Issue #51.
The controller action would build a new piece and version if the user
was logged in. Now it doesn't need to, so remove that functionality.
Clean up the specs accordingly.
Issue #51 - No new piece from root path

Logged in users no longer get a new piece form on the root path.
Issue #51 - Redirect to pieces path on login
Issue #51 - Pages index no longer shows piece form

No longer will a logged in user see a new piece form.
The main tinder page is shown to everyone. But a logged in user will not
see the sign in link.
Fix #51 - Redirect to pieces index after login