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.
You might consider using https://pypi.python.org/pypi/tabulate to print the table.

A quick example I worked up
```python
with open(csv_file) as fh:
    # WAS: contents = list(csv.DictReader(fh))
    contents = list(csv.reader(fh))

print tabulate.tabulate(contents, headers='firstrow', tablefmt='orgtbl')
```
gives
```
| Date       | Vendor          | Type   |   Quantity |   Price | Memo                  |
|------------+-----------------+--------+------------+---------+-----------------------|
| 2014-08-26 | United Airlines | Travel |          1 |     425 | Airfaire, IAD <-> SFO |
| 2014-08-27 | Yellow Cab      | Travel |          1 |      32 | Taxi to IAD           |
```