Revert "Fix #121:  Drop munging of 's~' prefix onto keys."
Merge pull request #274 from GoogleCloudPlatform/revert-259-121-drop_dataset_prefixes_in_keys

Revert "Fix #121:  Drop munging of 's~' prefix onto keys."
Basic regression/functional tests for datastore.

This commit only tests
- Adding a basic entity with an auto allocated ID, a
  given integer ID or a given name.
- Adding multiple entities via a transaction
- The given kind has no other entities (i.e. no false positives)

This commit adds
- A sample file to set environment variables for a test runner.
- A test runner.
- A utility for getting test relevant environ variables.