[WIP] Fix cpu percentage on multi threaded processes

[simonepri]

See the following:
https://nodejs.org/api/process.html#process_process_cpuusage_previousvalue
https://stackoverflow.com/a/16736599/3921589
#60 (comment)
#60 (comment)
#58
Unitech/pm2#3708

[codecov-io]

Codecov Report

Merging #61 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #61   +/-   ##
=======================================
  Coverage   92.57%   92.57%           
=======================================
  Files           9        9           
  Lines         283      283           
=======================================
  Hits          262      262           
  Misses         21       21

Impacted Files	Coverage Δ
lib/ps.js	95.34% <ø> (ø)	⬆️
lib/procfile.js	100% <ø> (ø)	⬆️
lib/wmic.js	89.28% <ø> (ø)	⬆️

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[soyuka]

These values measure time spent in user and system code respectively, and may end up being greater than actual elapsed time if multiple CPU cores are performing work for this process.

This should be true indeed. We may need to add this to the readme.