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.
Those functions are pretty short (average 10.5 lines each, with longest 22 and shortest 4).  Looking at them in the Sphinx-rendered doc, I don't think they need to be more tightly focused:  they are *all* about how to work with the API (rather than the verbose / messy script command line stuff).  E.g.:

![_get_employee](https://cloud.githubusercontent.com/assets/242750/4760054/841e8bcc-5aec-11e4-87e6-fdff77c894e1.png)
![create_report](https://cloud.githubusercontent.com/assets/242750/4760055/84218e62-5aec-11e4-9895-d956a773ae2f.png)