CAS SSO using CASTGC

[KenWhitesell]

I have recently implemented a custom LoginView (descending from CAS's LoginView) that supports the use of the TGT / CASTGC method of supporting SSO across different sites.

It works by overriding the get, post, and init_form methods of the original class, along with implementing new valid_tgt, process_tgt, gen_tgt and set_cookie methods. There's also a new TgtUser model created to store the currently-active TGT for an authenticated user.

Do you have any interest in possibly adding these to the project? I'd be happy to clean them up and submit them as a PR. There are a couple items that I would like to discuss before doing this regarding what elements should be made configurable through settings in the settings file.

I also have a couple of test Django projects built that can be run to demonstrate that it works as intended.