This includes the NUnit test adapter which allows NUnit tests to be run under VS without any extra plugins.
Unfortunate the compatibility tests using the abstract test fixture class show up as "external" tests, and aren't well presented - but they do run.

This includes fetching the VS unit test runner package, so that tests can be run from Visual Studio's Test Explorer.

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 could potentially be added back in later, but its use is limited and it's a pain in terms of support in PCL environments.
One use that has been highlighted is passing objects between AppDomains; we'd recommend passing a byte array explicitly and reparsing on the other side.

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