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.
This section is a bit long. I think we'd be better off focusing on the core `datastore` principles used and pointing to the source (which is a little bit circular, since we have to check it in before we can point to it).

But it would be possible if we checked in the example first and the docs second.