Add basic Xamarin.Mac support

Summary:
- Does not contain "magic" found in YogaKit yet, but enough to get started
- Simple test project showing use
- YGInteropSetLogger and YGNodeInit(IntPtr) were missing from native lib built by buck
Closes https://github.com/facebook/yoga/pull/278

Reviewed By: emilsjolander

Differential Revision: D4388480

Pulled By: splhack

fbshipit-source-id: a7387bb5b5554b6fce80d08c23e4fa18a4611cce

csharp/Mac/Facebook.Yoga.Mac.Test/Main.cs

@@ -0,0 +1,21 @@
+/**
+ * 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 AppKit;
+
+namespace Facebook.Yoga.Mac.Test
+{
+    static class MainClass
+    {
+        static void Main(string[] args)
+        {
+            NSApplication.Init();
+            NSApplication.Main(args);
+        }
+    }
+}