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>
Merge pull request #382 from SaveTheRbtz/helpers

lib/helpers: fixed is_valid_packet()
Merge branch 'master' of https://github.com/etsy/statsd