1. Port all existing AuthenticationHandler components to new AH interface.
   This afforded some opportunity for relatively minor refactoring.
2. Move CAS 3.0 AH interface, related 3.0 components, and
   LegacyAuthenticationHandlerAdapter to legacy module to provide
   transition path for third-party authentication handlers.
3. Remove CAS 2.x transitional components from legacy module.
4. Remove deprecated authentication-related components.