Net standard support

[LeeCampbell]

Another (4th?) attempt to target #17.
Recreated from scratch with the new VS2017 (15.2) IDE and csproj file formats.
While this was painful to make the port, the build seems so much more simple now.
dotnet cli (+ MSBuild under the hood) can now version, compile, test and package. 🎉

Key changes in this PR

• Finally made the jump to XUnit. In the end this only required minor changes. But the tooling is far better supported the nUnit and I just couldn't get nUnit to work as documented.
• Multi-target .net45 and netstandard1.3
• Version bump to 2.x.x
• Unit test use a custom spin wait class. Was seeing unpredictable Thread Sleep duration. This was part a misunderstanding on my behalf (thread.sleep will sleep at least as long as specified), and XUnit multithreaded runner taking all the threads (assumption). As a hammer, I have made the tests single threaded.
• Builds are now all using the dotnet cli. This means no powershell or fancy build scripts. See build.cmd to see how simple it is now.

Big thanks to @alhardy, @dtillman, @onovotny and ghuntley for their help (directly or indirectly) getting this PR ready.

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