Add logging for authentication handlers

Add logging for authentication handlers to the PolicyBasedAuthenticationManager.

This would also close PR #617.
Unless I am mistaken, at the moment, yes, e.getCause() will always return a cause so there should be no null pointer exception.  This is because it appears to be impossible to instantiate a PreventedException without giving it a cause.
https://github.com/Jasig/cas/blob/master/cas-server-core/src/main/java/org/jasig/cas/authentication/PreventedException.java

Thanks for bringing it up though, and please do double-check me.

Originally, I had a quick if-statement test to make sure e.getCause() didn't return null, but I removed it when I looked at the code linked to above.  If you think we're better off with me adding it back, not a problem.

Thanks!