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.