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
Gotcher. They are roughly equivalent, but if `MAX_FETCH = 10` and 7 results are returned, this would waste a query. However this is mostly irrelevant since only committers would use this script on test projects and even then they would only use it if they chose to after borking the datastore in a test.

It's not so big a deal either way, but I prefer not to use `while True`. It doesn't feel idiomatic