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
Probably just a matter of "regional accent":  `while True:` feels idiomatic to me when the loop can't be structured to have a "natural" test at the top.

Hmm, I might unroll the initial `curr_results = query.fetch()`, and then make the loop `while curr_results:`.  At any rate, this "matter of taste" shouldn't be a blocker to merge.