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
- [x] Make sure we don't bork builds on Mac / Linux
- [x] Handlebars the NuSpec file to drop in the current version from package.json
- [x] Codesign the installer in CI builds
- [ ] Make sure that the RcEdit hacking doesn't stomp the Squirrel-aware tag (needs atom/atom-shell#614)
Just a heads up, [underscore-plus](http://underscorejs.org/#template) supports templates and is available to build tasks.

https://github.com/atom/atom/blob/0098ac67efff2ba92e1a89f44d27b4dfedde0c0d/build/tasks/mkdeb-task.coffee#L9-L10

https://github.com/atom/atom/blob/0098ac67efff2ba92e1a89f44d27b4dfedde0c0d/resources/linux/debian/control.in#L1