stat_accum_input: add configurable namespaces
stat_accum_input: rename variables
stat_accum_input: Handle both new and legacy namespaces

+ Add tests for new namespaces
stat_accum_input: correctly calculate counter ratePerSecond

  According to etsy/statsd sources ratePerSecond is just
that counted amount in 1 second interval since we flush every TickerInterval
Calculated rate should just be sumOfCounters/TickerInterval
stat_accum_input: change method of testing empty strings

After second thought this way seems clearer, and 4 characters shorter
Merge pull request #313 from pchojnacki/statsd_changes

Statsd changes