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.
I don't understand the "circularity,:

WRT "core datastore principles"  the intent is to show a somewhat non-toy application using the API, rather than documenting either API reference or "concepts", both of which get documented elsewhere.  I agree that this doc could probably use more links to those places (like your URL for "ancestor" queries).