C# cleanup

Fixes for Mono

Convert back to using NUnit, which is now loaded via NuGet.

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.

Performance optimization for small messages without unknown fields

Add C# namespace to .proto files (e.g. descriptors, unit tests)

Remove ClsCompliant declarations from C# code (fix #318)

Field Presence for Protocol Buffer C# Proto3

Change to using xUnit for all unit tests, and fetch that via NuGet.

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

Convert C# projects to Portable Class Libraries

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.)

Tidying up the C# runtime project

We need to remove it from the generator too; I'll raise a github issue for that.

Move to a single solution file containing all of the C# projects, but no other solution folders - it's easier to edit those files outside VS than keep adding and removing them from the project.
The AddressBook protos have been regenerated (with a change to the example proto which I haven't included in this change - I'll wait for us to decide exactly what we're doing with namespaces before changing protos outside the csharp directory.

Note that now we've got Addressbook.cs which contains AddressBook and Addressbook classes. It's bad enough that we've got a class called AddressBook within a namespace of AddressBook (hard to get away from) but having things vary just by case is nasty.
This is more evidence that an option for renaming the file and descriptor class would be welcome. (A single option can probably handle both.)

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.

It is expected that third parties will generate service/RPC code themselves - see gRPC as an example.

1) Project files for different configurations - we're going to look at all this again, ideally to just have a single PCL-compatible build
2) ProtoGen - the C++ generator is now the only one we care about
3) Proto files - these are mostly duplicates (or older versions) of the ones in the common directories

Csharp update

First steps to making the C# runtime work with new codegen

(Having regenerated descriptor.proto relative to src, the earlier commented-out code checking that dependencies match may now be okay to uncomment again. Will experiment in later CLs.)

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