Adding script to enable clearing datastore.
Refactoring remove_kind to make while loop clearer.

Also stumbled upon other datastore gotchas (non-ancestor
deletes in a transaction) and added printing to indicate
what is being deleted.
Avoid extra URL-quoting of key name on upload.

Fixes #370.
Merge pull request #371 from tseaver/370-doubly_quoted_name_in_upload

#370: Avoid extra URL-quoting of key name on upload.
Merge pull request #369 from dhermes/clear-datastore-func

Adding script to enable clearing datastore.