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.
CAS-1418: Reverted change on logging aspect
CAS-1418: Javadoc update, and reused the credential instance post transform
CAS-1418: Renamed param name to be more explicit
CAS-1418: Pulled out uid into a username variable
Merge pull request #418 from Unicon/CAS-1418

CAS-1418: Allow LdapAuthnHandler to extend the authn subsystem