Convert both the full and lite runtimes (and json/xml serialization assemblies) to be Portable Class Libraries.
All referring projects are now .NET 4 client rather than .NET 3.5.
This commit also fixes up the ProtoBench app, which I'd neglected in previous commits. (Disentangling the two sets of changes would be time-consuming.)

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