To my surprise, executing generate_protos.sh used the version of Bash installed with Git for Windows by default.
After a few modifications to detect the most appropriate protoc to use, this worked pretty simply.
This change also:
- adds generation of the address book tutorial proto,
- fixes the addressbook.proto to specify proto2 explicitly (to avoid a warning from protoc; I don't think we want warnings...)
- fixes the addressbook.proto C# namespace (which I thought I'd done before, but apparently hadn't)
- includes the regenerated UnittestCustomOptions.cs apart from the DescriptorProtoFIle => Descriptor change

This commit includes changes to the C#-specific protos, and rebuilt versions of the "stock" protos.
The stock protos have been locally updated to have a specific C# namespace, but this is expected to change soon, so hasn't been committed.
Four areas are currently not tested:
1) Serialization - we may restore this at some point, possibly optionally.
2) Services - currently nothing is generated for this; will need to see how it interacts with GRPC
3) Fields beginning with _{digit} - see https://github.com/google/protobuf/issues/308
4) Fields with names which conflict with the declaring type in nasty ways - see https://github.com/google/protobuf/issues/309