lib/helpers: fixed is_valid_packet()

This commit introduces following changes:
 * Be liberal in what you recieve - do not rely on regexps for packet
   parsing but instead accept anything that can be parsed by JavaScript
   as a number e.g: +1e-17, -0511, 0xDEADbeef, etc
 * Accept both positive and negative counters with explicit / implicit
   sign
 * Provides more strict error checking then regexp, for example
   following strings match previously used '([\-\+\d\.]+' regexp:
   .
   .123.
   .\-+\0\+-.1-

Also while here added more tests.

Closes: #350 #357

Signed-off-by: Alexey Ivanov <SaveTheRbtz@GMail.com>
fixed a link

Link to datadog-backend was deprecated
Merge pull request #383 from themasch/patch-1

fixed a link (datadog backend)
Fix Typo.

indivdually -> individually
Merge pull request #386 from aleasoluciones/master

Fix Typo.
Updates winser to v0.1.6

A change to a dependency (sequence?) of winser v0.0.11 meant
thatthe service install script was no longer working.
Merge pull request #388 from ciaranj/update_winser

Updates winser to v0.1.6
move contributing information into CONTRIBUTING.md
bump to v0.7.1
Merge pull request #382 from SaveTheRbtz/helpers

lib/helpers: fixed is_valid_packet()
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