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/ViewController.cs

@@ -0,0 +1,54 @@
+using System;
+using CoreGraphics;
+using Facebook.Yoga;
+using UIKit;
+
+namespace Facebook.YogaKit.iOS.Sample
+{
+	public partial class ViewController : UIViewController
+	{
+		protected ViewController(IntPtr handle) : base(handle)
+		{
+			// Note: this .ctor should not contain any initialization logic.
+		}
+
+		public override void ViewDidLoad()
+		{
+			base.ViewDidLoad();
+			CreateViewHierarchy(View, View.Bounds.Size.Width, View.Bounds.Size.Height);
+		}
+
+		static void CreateViewHierarchy(UIView root, nfloat width, nfloat height)
+		{
+			root.BackgroundColor = UIColor.Red;
+			root.Yoga().IsEnabled = true;
+
+			root.Yoga().Width = (float)width;
+			root.Yoga().Height = (float)height;
+			root.Yoga().AlignItems = YogaAlign.Center;
+			root.Yoga().JustifyContent = YogaJustify.Center;
+
+			var child1 = new UIView { BackgroundColor = UIColor.Blue };
+			child1.Yoga().IsEnabled = true;
+			child1.Yoga().Width = 100;
+			child1.Yoga().Height = 100;
+
+			var child2 = new UIView
+			{
+				BackgroundColor = UIColor.Green,
+				Frame = new CGRect { Size = new CGSize(200, 100) }
+			};
+
+			var child3 = new UIView
+			{
+				BackgroundColor = UIColor.Yellow,
+				Frame = new CGRect { Size = new CGSize(100, 100) }
+			};
+
+			child2.AddSubview(child3);
+			root.AddSubview(child1);
+			root.AddSubview(child2);
+			root.Yoga().ApplyLayout();
+		}
+	}
+}