Update C# wrapper to support both desktop and UWP projects

Summary:
The C# wrapper project has been changed to a NETStandard (1.1) PCL allowing it
to be consumed by any project targeting .NET4.5 or greater including .NETCore
and UWP projects... The C# wrapper uses P/Invoke to call into the Native Yoga
DLL...

The "Yoga" C++ project has been updated to support ARM builds...

Added the ability to generate nuget packages for the C# wrapper that supports
copying the native DLLs to the target output directory.

Closes: https://github.com/facebook/yoga/pull/283

Reviewed By: splhack, emilsjolander

Differential Revision: D4381455

Pulled By: JoelMarcey

fbshipit-source-id: e6dc4190caa824ac63d5d5ee2eb4c7914efbb00f

csharp/Windows/Facebook.Yoga/project.json

@@ -0,0 +1,12 @@
+{
+  "dependencies": {
+    "Microsoft.NETCore.Portable.Compatibility": "1.0.1",
+    "NETStandard.Library": "1.6.0"
+  },
+  "frameworks": {
+    "netstandard1.1": {}
+  },
+  "language": "en",
+  "supports": {},
+  "version": "3.0.0-*"
+}