CAS authenticate after session expires

Adds a **CAS authenticate after session expires** option to plugin settings.  When checked, the next request will authenticate with CAS instead of redirecting to the non-CAS login page.  When unchecked (the default), it retains the prior behavior.  See issue #7.

It works by overriding the `session_expiration` filter in `ApplicationController`.  When it detects an expired session, it resets the session and falls through.  The subsequent login check will trigger CAS authentication.

I haven't added a French translation for the new setting in `fr.yml`.