SIGNUP/LOGIN VIA FACEBOOK

Added config/facebook.yml but git-ignored (see email for a working example). Only works for http://localhost:8080!!!

Added config/initializers/facebook.rb, that parses the YAML file above.

Added the omniauth-facebook gem, to support Facebook authentication/authorisation.

Added a route to support Facebook callbacks.

Added method in PublicController to handle Facebook callback (where the user is sent from FB after successful authorisation).

Created migration to add facebook_uid to users.

Added User#create_from_facebook method, that will create a user based on data that Facebook gives us.

Added skip_password_validation accessor for a User, so we can create users with no password (in res, Facebook users).