Prepared load (-l N) support for windows.

Inspired by:
http://stackoverflow.com/questions/23143693/retrieving-cpu-load-percent-total-in-windows-with-c
Fixed/improved -l N documentation For windows

On windows "system load" is not so obvious term, added line explaining N
argument in -l N
Fixes for windows CalculateProcessorLoad

- Fixed bad logic condition,
- Added comment to clarify
Small usage info fix for windows
Improved load calculation

Added code to gracefully handle:
1. Call to CalculateProcessorLoad with not incremented ticks (fast calls
to GetSystemTimes can result same results),
2. Smooth/filter load estimation for consecutive calls.
Fixed naming convention in GetLoadAverage support functions.
Changed implementation to provide load from 0 to ProcessorCount

This makes this implementation more consisten with POSIX load avarage.
Remove extra info from Usage()

As now behavior is similar on on platforms, this can probably be removed
Merge pull request #798 from kwesolowski/master

Provided Basic Support for -l N option on windows platform.
add some parens to silence a gcc warning
amend HACKING.md with instructions for Arch Linux
Merge pull request #802 from martine/parens

add some parens to silence a gcc warning
Merge pull request #803 from martine/arch

amend HACKING.md with instructions for Arch Linux