Datastore narrative example

Add an example "expense reporting" application (command-line only), and use it as the basis for a bit of narrative documentation showing how to use the gcloud-python library to a "real-world" app.

Note that the app's scripts actually run, allowing CRUD of hierarchical employee, expense report, and expense item entities:  they could be used for further regression testing.
RE: Circular, I just meant it'd be hard to provide a link to something before it's checked in.

The main thrust of my comment is that here (and in the sections that follow),  printing every line of code via `literalinclude` causes the doc to drag on. We'd be better off with a choice selection of relevant lines to draw focus. If people wanted a deep dive into the full source, we can then provide a link. (This is somewhat an argument for breaking this off into two separate code only and docs only changes.)