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.
Merge pull request #273 from dhermes/functional-test-part1

Functional test part1
Don't copy key's datset ID to the protobuf.

The back-end already knows it, and it is actually different than the
'project ID' we are assigning to the dataset as its ID.

Fixes #121.
Remove direct 'Key' -> 'Dataset' relationship.

Replace it with an (optional) '_dataset_id' attribute, which is None for
newly-created keys, or the more-or-less opaque value set by the backend
for keys fetched from it.

Add direct 'Entity' -> 'Dataset' relationship (rather than indirecting
through the entity's key.

Addresses @pcostell's remarks on #259:

https://github.com/GoogleCloudPlatform/gcloud-python/pull/259#issuecomment-59971468