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/YogaKit.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Runtime.InteropServices;
+using Foundation;
+using UIKit;
+
+namespace Facebook.YogaKit
+{
+	public static partial class YogaKit
+	{
+		static NSString YogaNodeKey = new NSString("YogaNode");
+
+		public static IYogaLayout Yoga(this UIView view)
+		{
+			return YogaLayoutNative(view);
+		}
+
+		static IYogaLayout YogaLayoutNative(UIView view)
+		{
+			var yoga = ObjCRuntime.Runtime.GetNSObject(objc_getAssociatedObject(view.Handle, YogaNodeKey.Handle)) as YogaLayout;
+			if (yoga == null)
+			{
+				yoga = new YogaLayout(view);
+				objc_setAssociatedObject(view.Handle, YogaNodeKey.Handle, yoga.Handle, AssociationPolicy.RETAIN_NONATOMIC);
+			}
+			return yoga;
+		}
+
+		[DllImport("/usr/lib/libobjc.dylib")]
+		static extern void objc_setAssociatedObject(IntPtr @object, IntPtr key, IntPtr value, AssociationPolicy policy);
+
+		[DllImport("/usr/lib/libobjc.dylib")]
+		static extern IntPtr objc_getAssociatedObject(IntPtr @object, IntPtr key);
+
+		enum AssociationPolicy
+		{
+			ASSIGN = 0,
+			RETAIN_NONATOMIC = 1,
+			COPY_NONATOMIC = 3,
+			RETAIN = 01401,
+			COPY = 01403,
+		}
+	}
+}