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.Sample/AppDelegate.cs

@@ -0,0 +1,23 @@
+using Foundation;
+using UIKit;
+
+namespace Facebook.YogaKit.iOS.Sample
+{
+	[Register("AppDelegate")]
+	public class AppDelegate : UIApplicationDelegate
+	{
+		public override UIWindow Window
+		{
+			get;
+			set;
+		}
+
+		public override bool FinishedLaunching(UIApplication application, NSDictionary launchOptions)
+		{
+			return true;
+		}
+
+
+	}
+}
+