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".