CAS-1418: Allow LdapAuthnHandler to extend the authn subsystem

The current ldap authentication handler does not extend the `AbstractUsernamePasswordAuthenticationHandler`, because the parent API does not provide support for warning messages to be returned upstream. This commit slightly re-arranges the API to allow each handler to submit warning messages, so that ldap authn handler can also participate in the chain.

Also, as was discussed earlier, the pp-configuration is now moved upstream into the AbstractUsernamePasswordAuthenticationHandler, so that other handlers may be able to benefit from creating policies.

The changes were previously discussed in this pull request, https://github.com/Jasig/cas/pull/397
and are intended for a 4.0 GA release.