travis => windows builds on EC2s

Automates node-cpp11 builds on AWS Windows EC2s.

- [ ] Make + upload source tarball... from windows? (https://github.com/mapbox/node-cpp11/blob/master/.travis.yml#L39-L40)
- [ ] Sign node.exe binaries after build
- [ ] No debug/error log from EC2s currently
- [ ] x64 builds only atm, need to add (?) 32-bit builds

cc @springmeyer @BergWerkGIS
does `cache:` work now in public repos? http://docs.travis-ci.com/user/caching/ still says only private.