Provide a method to completely backup and restore the datastore

[dexw25]

This would be helpful for scenarios like #47 where there is currently no recourse if things go horribly wrong

[ZMYaro]

While I agree there may be value in building our own custom backup system at some point, we currently have a backup on Google Cloud Storage as described in https://cloud.google.com/appengine/docs/standard/python/console/datastore-backing-up-restoring.