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).
There needs to be some way for the application to set the dataset from external configuration (e.g., a web server's INI file), so that we encourage the use of the free functions.