Add Google Protobuf as serializer

[sblasetti]

Hello, thanks a lot for making this test of serializers available to all of us! I needed to compare performance of Google Protobuf with protobuf-net and others. Thought it might be useful to others having it included here.

• Add Google Protobuf as one of the supported serializers.
• Add .proto contract file and generated code.
• Add Data generation methods and include the new serializer in the list of serializers to test.

[Alois-xx]

Thanks for adding this one!