Fixing docstring on Dataset.get_entities.
Merge pull request #450 from dhermes/fix-docstring-error

Fixing docstring on Dataset.get_entities.
Fix regression test failure introduced in PR #449.
Merge pull request #452 from tseaver/449-fix_transaction_borked_in_449

Fix regression test failure introduced in PR #449.
Adding helper methods for implicit datastore environ.

By setting two environment variables

- GCLOUD_DATASET_ID
- GOOGLE_APPLICATION_CREDENTIALS

the user can call convenience methods in `gcloud.datastore` directly
without worrying about auth or the name of the dataset. The
goal is that in places like App Engine and Compute Engine, the
dataset ID can be implied without **any** user intervention.

Partially addresses #337.

NOTE: This still needs to be documented, but it's unclear
      where is appropriate. We also need to have documentation
      for auth (outside of CONTRIBUTING.md).