Clean up & simplify the build process

[stakx]

Among other things:

• Check whether the Git submodule Common is still required.
• Try unifying the .NET 4.5 project and the .NET Standard 1.3 project into a single SDK .csproj.
• See which project.json and packages.config are superfluous and which are still required.
• Also clean up the unit test project, making sure tests are still run once for each target.
• Get GitLink SourceLink working and include symbols (PDB) in package.
• Remote the outdated release notes from Moq.nuspec. Instead, link to the changelog file.
• Try to integrate Moq.nuspec into the unified .csproj. (If we want to include the PDBs in the package, it appears we'd need to apply some tricks even with the new .csproj format, so at this time I'm not convinced that getting rid of Moq.nuspec would be a large win.)
• Possibly get rid of the Visual Basic test project. It's nearly not used at all but possibly slows down test discovery. (It contains a test that couldn't be written in C#, decide later what to do with it.)
• Include Pex unit test project in solution. (It's hopelessly updated, decide later what to do with it.)