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).
Making cached dataset ID require explicit interaction.

Incorporates feedback from @tseaver. Also
- Fixing bad tests that accidentally leave side-effects.
- Making the method to set the default dataset public.
Merge pull request #444 from dhermes/implicit-dataset-from-environ

Implicit dataset from environ