protogen-net: generate C# from .proto files

This is an old build of Marc Gravell's Protobuf-net's protogen tools, that current versions of his package no longer include (for some reason).

You may want to look at his current version: https://github.com/mgravell/protobuf-net or Google's version instead: https://github.com/google/protobuf/tree/master/csharp