Providing a multi-put example, illustrating the use of Proto generation/conversion API.

Provide an example on how to use:
EndpointsModel.ProtoModel()
EndpointsModel.ProtoCollection()
EndpointsModel.ToMessage()
EndpointsModel.FromMessage()
to generate ProtoRPC classes to use with Endpoints API.
The example is constructed as a "batch-insert" API that takes in a "proto collection" to do batch insert into NDB and return the entities inserted as another "proto collection".
Because this handler is decorated by endpoints.method not by EndpointsModel.method
endpoints.method does not provide this keyworded argument (as documented in https://cloud.google.com/appengine/docs/python/endpoints/create_api).