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
I guess I'm thinking that breaking out of the loop when `query.fetch()` returns an empty set is the right boundary (rather than the guess based on `len(curr_results) == FETCH_MAX`:  basically, the same as finding EOF in a file.