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.
If the app were really being used, that would be great.  However, I'd hate to introduce a dependency on a non-stdlib module here in what is basically a tutorial:  the context-switch isn't worth the prettier output, I think.