Add `atom.services` API

This depends on the [`service-hub`](https://github.com/atom/service-hub) library, but I wanted to make the documentation Atom-specific in the project so I'm defining a trivial subclass in core. It's kinda weird, but gets the job done. Open to suggestions as to other approaches. Check out the documentation at the top of the `ServiceHub` class on this branch for more details about the design.