support Atom Shell as a runtime

The following additions have to be implemented to support Atom Shell and fix #110:

* [x] add atom-shell-related versioning
* [ ] add atom-shell-related [building of native modules](https://github.com/atom/atom-shell/blob/master/docs/tutorial/using-native-node-modules.md#the-node-gyp-way)
* [ ] add atom-shell-related docs (mostly in `README`)

**Note:** this pull request also fixes #111 because is based on its branch.