travis => windows builds on EC2s

Automates node-cpp11 builds on AWS Windows EC2s.

- [x] Make + upload source tarball from linux ~~from windows? (https://github.com/mapbox/node-cpp11/blob/master/.travis.yml#L39-L40)~~
- [x] Sign node.exe binaries after build
- [x] ~~No debug/error log from EC2s currently~~ punt, use `[publish debug]` in commit message
- [x] x64 builds only atm, need to add (?) 32-bit builds - https://github.com/mapbox/node-cpp11/commit/3e693cc52f5a4b7bcebcd3177227c28246fe47a2

cc @springmeyer @BergWerkGIS