Add CAS authentication via the CAS strategy of omniauth

Issues with this that I can think of:
* It only uses the :host attribute for setting up cas. https://github.com/dlindahl/omniauth-cas  This will work for CAS setups that do not do anything odd like:
  * using cas on the end of the url
  * odd port numbers instead of the default 443
* No tests other than it work for me.

These could be worked around if I could figure out how to optionally set variables for the provider.