Add editorial workflow support to test backend

[erquhart]

The in-memory test backend is used by the example project for local development of this project, and does not currently support the editorial workflow. To work on and test the editorial workflow locally, a developer has to create a separate test project, which isn't ideal. The test backend implementation should be updated to support the editorial workflow.

[Benaiah]

https://github.com/creationix/js-git may be useful for this.