added CAS authentication client

The CAS authentication client is implemented and will currently will force authentication with Georgia Tech's CAS server when you visit any page. However, you can't actually use the authentication server until we register our application with OIT's Identity Management team.
See here: http://www.oit.gatech.edu/sites/default/files/OIT_IDM_Policy.pdf

"Any applications in Georgia Tech network space may use CAS or Active Directory to authenticate users without registering the application with OIT. However, if the application exists outside of Georgia Tech (e.g. a 3rd party service or within a non-gatech.edu domain), then the IDM team will need to be consulted to register the service to use one of the authentication services."