expose requiredAuthenticationMethod in login JSP, correct bug selecting prior Authentication

Exposes the `requiredAuthenticationMethod` to the `casLoginView.jsp` so that, optionally and as shown in the example, an adopter's login view can express the required authentication method or otherwise help set user expectations about the context in which they are seeing the main username and password form (as prelude to being prompted for an additional credential).

Also corrects a bug in selecting out the relevant prior Authentication and adds/improves some JavaDoc.