stat_accum_input: cleanup Flush() implementation

+ prepare code for setting custom namespaces
stat_accum_input: remove false timer messages

  When timer request with no timer values was processed Heka generated
false(?) timer messages as if timer 0 was sent. etsy/statsd seems to ignore
such requests.
stat_accum_input: fix timer calculation

+ added test to validate calculation results
stat_accum_input: use InputRunner.LogError instead of log.Println
Merge pull request #301 from pchojnacki/statsd_cleanup

stats_accum_input cleanup