Add YogaKit Shared and iOS version

Summary:
Using shared code for reuse in other platforms based on iOS native implementation.

Adds YogaKit sample.

Adds YogaKit tests (same as objc).

```
YogaKitTest : 80 ms
Facebook.YogaKit.iOS.Tests.exe : 81 ms
Tests run: 11 Passed: 8 Inconclusive: 0 Failed: 3 Ignored: 1
```

Since we don't have extension properties we need to go with a extension method to get access to the YogaLayout .

I m also not sure this is leak free yet, would love some help with testing and feedback about view/node lifecycle
Closes https://github.com/facebook/yoga/pull/336

Reviewed By: splhack

Differential Revision: D4415027

Pulled By: emilsjolander

fbshipit-source-id: c88328212426c3200e6f0c48cda594cd2c432065

csharp/iOS/Facebook.YogaKit.iOS.Tests/Main.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Linq;
+using System.Collections.Generic;
+
+using Foundation;
+using UIKit;
+
+namespace Facebook.YogaKit.iOS.Tests
+{
+    public class Application
+    {
+        // This is the main entry point of the application.
+        static void Main(string[] args)
+        {
+            // if you want to use a different Application Delegate class from "UnitTestAppDelegate"
+            // you can specify it here.
+            UIApplication.Main(args, null, "UnitTestAppDelegate");
+        }
+    }
+}