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/Main.cs

@@ -0,0 +1,13 @@
+using AppKit;
+
+namespace Facebook.Yoga.Mac.Test
+{
+	static class MainClass
+	{
+		static void Main(string[] args)
+		{
+			NSApplication.Init();
+			NSApplication.Main(args);
+		}
+	}
+}