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/UnitTestAppDelegate.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Linq;
+using System.Collections.Generic;
+
+using Foundation;
+using UIKit;
+using MonoTouch.NUnit.UI;
+
+namespace Facebook.YogaKit.iOS.Tests
+{
+    // The UIApplicationDelegate for the application. This class is responsible for launching the 
+    // User Interface of the application, as well as listening (and optionally responding) to 
+    // application events from iOS.
+    [Register("UnitTestAppDelegate")]
+    public partial class UnitTestAppDelegate : UIApplicationDelegate
+    {
+        // class-level declarations
+        UIWindow window;
+        TouchRunner runner;
+
+        //
+        // This method is invoked when the application has loaded and is ready to run. In this 
+        // method you should instantiate the window, load the UI into it and then make the window
+        // visible.
+        //
+        // You have 17 seconds to return from this method, or iOS will terminate your application.
+        //
+        public override bool FinishedLaunching(UIApplication app, NSDictionary options)
+        {
+            // create a new window instance based on the screen size
+            window = new UIWindow(UIScreen.MainScreen.Bounds);
+            runner = new TouchRunner(window);
+
+            // register every tests included in the main application/assembly
+            runner.Add(System.Reflection.Assembly.GetExecutingAssembly());
+
+            window.RootViewController = new UINavigationController(runner.GetViewController());
+
+            // make the window visible
+            window.MakeKeyAndVisible();
+
+            return true;
+        }
+    }
+}