Tidy up test directory
Set up tests using Minitest::Spec and FactoryGirl
Set up Procfile
Set up Omniauth with the Google OAuth2 provider
First pass at a user model

Users are created from an auth_hash returned from Omniauth, and are 
restricted to the digital.cabinet-office.gov.uk email address.
Add a SessionsController for authentication
Add current_user and user_signed_in? helper methods
Add flash messages