add a new namespace option for stats in the statsd-compliant processor

The legacy namespace is still used by default. The new namespace is
almost the same as the non-legacy defaults in the Etsy project. The one
difference is the the suffix ".value" is added to gauges in the
txstatsd implmentation.
Merge pull request #1 from bicycleday/master

add a new namespace option for stats in the statsd-compliant processor