refs #9 - CAS-login for redmine with public content.

I think this is an acceptable fix for #9 (enable CAS logins if your redmine installation has public content).

If your redmine is "authentication required", nothing changes. Otherwise, the login page will show a link 'login using CAS'.

All this probably needs some polishing. I am new to redmine plugins, and to ruby in general.
Please remove this comment.