Oauth authentication

Someone accesses the site:

- check if there is a session token (you need to configure the path in settings.yml)
- if there is none: go to google and demand access to user details (email)
- if the user gives access, we check if the address is @liip.ch  (need to make that configurable later)
- if it is we allow the rest, otherwise we stop and print "access denied"

to make the google oauth callback work locally you have to use a public domain, e.g. presence.example.com

set that in /etc/hosts and point it to the vagrant box IP