Add basic Xamarin.Mac support

- Does not contain "magic" found in YogaKit yet, but enough to get started
- Simple test project showing use

csharp/Facebook.Yoga/mac/Facebook.Yoga.Mac.Test/AppDelegate.cs

@@ -0,0 +1,22 @@
+using AppKit;
+using Foundation;
+
+namespace Facebook.Yoga.Mac.Test
+{
+	[Register("AppDelegate")]
+	public class AppDelegate : NSApplicationDelegate
+	{
+		public AppDelegate()
+		{
+		}
+
+		public override void DidFinishLaunching(NSNotification notification)
+		{
+		}
+
+		public override void WillTerminate(NSNotification notification)
+		{
+			// Insert code here to tear down your application
+		}
+	}
+}