Normalize emulator support

[tseaver]

Follow the google-cloud-firestore implementation:

• Drop use of the google.cloud.environment_variables indirection for the environment variable names driving emulation (likely also DISABLE_GRPC and GCD_DATASET.
• Document the environment variables locally, as well as how to run the datastore emulator.
• If the DATASTORE_EMULATOR_HOST environment variable is set, use an "insecure channel", to avoid requiring credentials (see Datastore: Client requires authentication even when connecting to the datastore emulator #11). That won't work, as the datastore client straddles both HTTP and gRPC. Instead, use AnonymousCredentials if none are supplied.