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/ApiDefinition.cs

@@ -0,0 +1,13 @@
+using System;
+
+using AppKit;
+using Foundation;
+using ObjCRuntime;
+using CoreGraphics;
+
+namespace Facebook.Yoga.Mac
+{
+	// We're actually abusing the Xamarin.Mac binding project
+	// to bundle in the static lib for us. There is no objective-c
+	// lib to bind, but the build will fail without ApiDefinition.cs
+}