Client cert checker *seems* to be working.  Needs more testing ...

I seem to be able to specify multiple checkers.  Here is an example command line:

  $ twistd -n cas \
  > --auth client_cert:subject_part=emailAddress:transform=strip_domain \
  > --auth file:./cas.passwd \
  > --ticket-store memory_ticket_store \
  > --ssl -k ssl/key.pem -c ssl/cert.pem --addCA ssl/ca.cert.pem --verify-client-cert
  > --realm basic_realm

This lets me log in using trust authentication (client cert) with my test client.
My browser, which does not present a client cert, gets the login page, and I can
log in using a username/password.

I'd like to clean up the logging of an error stack trace that says the browser
didn't present a certificate-- that could be cleaner.

I'd also like to have CAS respond in a more appropriate way if there are no
username/password cred checkers-- showing the login screen doesn't seem appropriate
in that case.