Tickets: Adapting backends from the KTHse/django-cas2 fork.

This allows the cas_tgt table to be populated.

Cf. https://github.com/KTHse/django-cas2/blob/master/django_cas/backends.py
Debugging: Exposing reason for CasTicketException in get_proxy_ticket_for().
Merge pull request #16 from amiato/proxy-ticket-fixes

Proxy ticket fixes