Build a Squirrel installer as part of CI build

This PR kicks off the work we need to do in order to create an installer / update framework for Atom on Windows:

## TODO:

- [x] Get an installer building
- [ ] Handlebars the NuSpec file to drop in the current version from package.json
- [ ] Make sure that the RcEdit hacking doesn't stomp the Squirrel-aware tag (needs atom/atom-shell#614)
- [ ] Codesign the installer in CI builds