Switch the default of Statsd counter representation in Librato.

This will now send the aggregate inc/dec values to Librato as a
gauge. This requires less tracking resources at our statsd backend and
generally works better on the Librato side.
Add configuration section describing how to not zero-fill client side.
Update our node dependency to match StatsD.
Update required StatsD version.
Merge pull request #23 from librato/feature/counters_as_gauges

Publish statsd counters as Librato gauges