Authenticate user using CAS.

You have to set it up correctly in conf/bootstrap_plugins.php using the following keys in $PLUGINS["AUTH_DRIVER"]["OPTIONS"] array:

  CAS_SERVER: hostname of the CAS server (example: login.example.com)
  CAS_URI: URI where CAS server is installed on (example: /cas)
  CAS_PORT: port where CAS server is listening on waiting for connections
  USERS_FILEPATH: Inherited from auth.serial. File where user data is stored