Fix #260: break datastore cycles by moving 'from_protobuf' classmethods

Third time's the charm for #260 in datastore:

'gcloud.datastore._helpers' -> 'gcloud.datastore.helpers' (now API).

'gcloud.datastore.entity.Entity.from_protobuf' classmethod -> 'gcloud.datastore.helpers.entity_from_protobuf'.

'gcloud.datastore.key.Key.from_protobuf' classmethod -> 'gcloud.datastore.helpers.key_from_protobuf'.