Fixes issue #72

Save environment variables in binding.gyp variables resolves issue with
node-gyp not being able to resolve the path right on some Windows
environments. mdns-tests are running successful. "npm install mdns"
working under Windows 8.1 x64 with Bonjour SDK from Apple installed and
using Visual Studio 2013 Professional.
Merge pull request #93 from reppners/master

Fixes issue #72