Add get_proxy_ticket

[dstegelman]

Should this be apart of django-cas?

def get_proxy_ticket(username):
    from cas.models import Tgt
    from django.core.exceptions import ObjectDoesNotExist

    try:
        tgt = Tgt.objects.get(username=username)
        proxy_ticket = tgt.get_proxy_ticket_for(getattr(settings, 'TICKET_SERVICE_URL', None))

        return proxy_ticket
    except ObjectDoesNotExist:
        return None