Fix #each_metric and #get

MetricRegistry#allMetrics was replaced by #metrics
Fix metrics registration, without options

Metrics registration changed and no longer accepts options like biased,
time_unit, etc. So, the options have simply been dropped. (I think the
effects of the options can be achieved via reporters)
Metric names are now simply strings

They seem to be automagically scoped as well.
Fix error_translation

IllegalArgumentException is now thrown when trying to redeclare a metric
as another type.
Remove event_type

This concept seems to have been removed completely as far as I can tell.
max, min, mean, std_dev moved to Snapshot

They are no longer methods on histograms and meters, but on the
snapshot.

The #sum method does not seem to exist even on the snapshots -- so I
removed it from the implementation here as well.
Rename Yammer -> Codahale
Update JMX reporter

slf4j is required for JMX Reporter
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