Refactoring and reformating, caching ResourceContainers
Refactoring and reformating, caching ResourceContainers

Moved message_fields creation to a separate method.
Reformated to make lines shorter.
Caching ResourceContainers (separate from proto models and collections)

GoogleCloudPlatform/endpoints-proto-datastore,maxandron/endpoints-proto-datastore,mnieper/endpoints-proto-datastore,jbergant/endpoints-proto-datastore,dhermes/endpoints-proto-datastore