Implicit dataset from environ

This has #436 as a diff-base, so feel free to hold off review until it is merged.

This adds 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).