Enhance the map-reduce Job end notifier to be able to notify the given URL via a custom class

Currently org.apache.hadoop.mapreduce.v2.app.JobEndNotifier allows a very limited configuration on how the given Job end notification URL should be notified. We should enhance this, but instead of adding more mapreduce.job.end-notification.* properties to be able to configure the underlying HttpURLConnection, we should add a new property so users can use their own notifier class.