Xamarin iOS Support

Summary:
Since there's a couple of changes after last month pr of #280 , here's a new pr for Xamarin support #276 , the idea is to add Xamarin iOS, Mac and Android support.

- Add Xamarin iOS support for Yoga, by providing a wrapper for the native lib.
- Add Xamarin iOS Unit testing project
~~- Adds a YogaKit shared implementation in c#.~~
~~- Adds a YogaKIt iOS implementation.~~
~~- Adds Yoga/YogaKit iOS sample application~~

Facebook.Yoga.iOS.Tests.exe : 169 ms
Tests run: 114 Passed: 114 Inconclusive: 0 Failed: 0 Ignored: 0

![simulator screen shot 6 jan 2017 02 05 15](https://cloud.githubusercontent.com/assets/1235097/21705017/f851e9d8-d3b4-11e6-9c44-646dab559643.png)
Closes https://github.com/facebook/yoga/pull/324

Reviewed By: emilsjolander

Differential Revision: D4401408

Pulled By: splhack

fbshipit-source-id: ecdae7967060361bef2bc25a5ef759cb9a957322

csharp/iOS/Facebook.Yoga.iOS.Tests/UnitTestAppDelegate.cs

@@ -0,0 +1,53 @@
+/**
+ * Copyright 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the license found in the
+ * LICENSE-examples file in the root directory of this source tree.
+ */
+
+using System;
+using System.Linq;
+using System.Collections.Generic;
+
+using Foundation;
+using UIKit;
+using MonoTouch.NUnit.UI;
+
+namespace Facebook.Yoga.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;
+        }
+    }
+}