tests: improved tests for lib/helpers
lib/helpers: added support for null 'ms' metrics.

Pointed out by: jeffyip in #382 [1]

[1] https://github.com/etsy/statsd/pull/382#discussion_r9887857
Merge pull request #397 from SaveTheRbtz/helpers

lib/helpers: added support for zero 'ms' metrics
Clarify use of backends with example
Merge pull request #400 from pburkholder/pdb_clarify_backends

Clarify use of backends with example
Merge pull request #1 from etsy/master

Re-sync Fork Repository