Allowing ChocolateyInstall from System & Proc Env

Based on the discussion in the google group
https://groups.google.com/forum/?fromgroups#!topic/chocolatey/84nMqjleMcI

This patch adds support for System and Proc based environment variable when installing chocolatey.

This still sets the path only for the user, but I am assuming if using system or proc the person installing will manually override the path.