BUG: Fixes #69 by adding a check to see if the form has been processed

This fixes the issue of being able to browse to the form submission confirmation page by appending finished to the URL.
It works by setting a session variable which contains the Security ID just before redirecting to the confirmation page.
When the finished method is called it checks if the session variable is set and that it matches the SecurityID if these criteria are not met it redirects back to the form page.