Version 0.4.2 (Pre M1)

• Adding get_for_service_account_json function to allow JSON
  service accounts without GOOGLE_APPLICATION_CREDENTIALS.
• Adding support for GCD datastore dev server.
• Adding Dataset class to act as a factory containing a
  preset connection and dataset ID. (Existed several releases
  ago in a different form.)
• Adding lazy loading support for default dataset ID and
  connection in datastore package. This allows auth to be done
  without needing to call datastore.set_defaults() provided
  the correct environment variables are set.