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.Universal.Tests/project.json

@@ -0,0 +1,16 @@
+{
+  "dependencies": {
+    "Microsoft.NETCore.UniversalWindowsPlatform": "5.2.2"
+  },
+  "frameworks": {
+    "uap10.0": {}
+  },
+  "runtimes": {
+    "win10-arm": {},
+    "win10-arm-aot": {},
+    "win10-x86": {},
+    "win10-x86-aot": {},
+    "win10-x64": {},
+    "win10-x64-aot": {}
+  }
+}