Issue 246 - C++ Tests do not compile in VS2013

@CharliePoole, can you pull this down and see if the C++ tests build properly on your machine? I changed the project files so that they set the toolset based on the version they find. It should work for VS2012 and 2013 now. I forgot we don't support 2010 anymore because of async.

The changes to nunit.sln are removing of extra platforms that were added by mistake. We should only ever be building AnyCPU, but Visual Studio likes to add the others if you are not careful. Those changes should be merged to master even if this toolset fix doesn't work.

If this doesn't work, we may want to consider a separate solution for the C++ test assemblies and check the binaries into a lib folder.