This commit changes the structure of Verspaetung pretty dramatically to allow
for the registering of Gauges for the various offsets
With this change the KafkaPoller is pushing the latest offsets into a map and the
ZK consumer tree watchers are pushing consumer offsets into a separate map.
References #17
This gives us two kinds of AbstractTreeWatcher instances, those that watch
special-case subtrees (e.g. the BrokerTreeWatcher) and then those which need to
watch and report Kafka consumer offset information (e.g. StandardTreeWatcher)
References #9