Adding platform detection, in prep for supporting more OS

Adds platform detection and support for npme user.
I've tested standing up about 5 Trusty servers, and 5 Centos 6.5 servers with the npm@2.0.0-beta.3 instructions -- which are what we have elsewhere in documentation.

I'd prefer to hold off on updating instructions/scripts to use npm@v2.0rc, until after we land this massive set of changes.