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/Assets.xcassets/AppIcon.appiconset/Contents.json

@@ -0,0 +1,68 @@
+{
+  "images": [
+    {
+      "filename": "AppIcon-16.png",
+      "size": "16x16",
+      "scale": "1x",
+      "idiom": "mac"
+    },
+    {
+      "filename": "AppIcon-16@2x.png",
+      "size": "16x16",
+      "scale": "2x",
+      "idiom": "mac"
+    },
+    {
+      "filename": "AppIcon-32.png",
+      "size": "32x32",
+      "scale": "1x",
+      "idiom": "mac"
+    },
+    {
+      "filename": "AppIcon-32@2x.png",
+      "size": "32x32",
+      "scale": "2x",
+      "idiom": "mac"
+    },
+    {
+      "filename": "AppIcon-128.png",
+      "size": "128x128",
+      "scale": "1x",
+      "idiom": "mac"
+    },
+    {
+      "filename": "AppIcon-128@2x.png",
+      "size": "128x128",
+      "scale": "2x",
+      "idiom": "mac"
+    },
+    {
+      "filename": "AppIcon-256.png",
+      "size": "256x256",
+      "scale": "1x",
+      "idiom": "mac"
+    },
+    {
+      "filename": "AppIcon-256@2x.png",
+      "size": "256x256",
+      "scale": "2x",
+      "idiom": "mac"
+    },
+    {
+      "filename": "AppIcon-512.png",
+      "size": "512x512",
+      "scale": "1x",
+      "idiom": "mac"
+    },
+    {
+      "filename": "AppIcon-512@2x.png",
+      "size": "512x512",
+      "scale": "2x",
+      "idiom": "mac"
+    }
+  ],
+  "info": {
+    "version": 1,
+    "author": "xcode"
+  }
+}

csharp/Facebook.Yoga/mac/Facebook.Yoga.Mac.Test/Assets.xcassets/Contents.json

@@ -0,0 +1,6 @@
+{
+  "info" : {
+    "version" : 1,
+    "author" : "xcode"
+  }
+}