Adding script to enable clearing datastore.

I peeled this off of an ideas branch of been working with:
https://github.com/dhermes/gcloud-python/tree/better-auth-story
Yes, there will be cases where an empty query occurs after the number of results remain is exactly `FETCH_MAX`, which wastes quota, etc.

See the [comment][1] in `datastore.query` which links to #280 

The `more_results` bit is sadly broken.

[1]: https://github.com/GoogleCloudPlatform/gcloud-python/blob/e3f6ae41e4378f06cd2d2c20613443a85283d65f/gcloud/datastore/query.py#L340