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/ViewController.cs

@@ -0,0 +1,83 @@
+using System;
+
+using AppKit;
+using Foundation;
+using CoreGraphics;
+
+namespace Facebook.Yoga.Mac.Test
+{
+	public static class NSViewYogaExtensions
+	{
+		public static void ApplyYogaLayout(this NSView view, YogaNode n)
+		{
+			view.Frame = new CGRect(n.LayoutX, n.LayoutY, n.LayoutWidth, n.LayoutHeight);
+			// This assumes your YogaNode and NSView children were inserted in same order
+			for (int i = 0; i < n.Count; ++i) {
+				YogaNode childNode = n[i];
+				view.Subviews[i].Frame = new CGRect (childNode.LayoutX, childNode.LayoutY, childNode.LayoutWidth, childNode.LayoutHeight);
+			}
+		}
+	}
+	
+	public partial class ViewController : NSViewController
+	{
+		public ViewController(IntPtr handle) : base(handle)
+		{
+		}
+
+		public override void ViewDidLoad()
+		{
+			base.ViewDidLoad ();
+
+			NSImage image = NSImage.ImageNamed (NSImageName.TrashFull);
+
+			NSView root = CreateViewHierarchy (image);
+
+			var rootNode = CalculateLayout (image.Size);
+
+			root.ApplyYogaLayout (rootNode);
+
+			View.AddSubview (root);
+		}
+
+		static NSView CreateViewHierarchy (NSImage image)
+		{
+			var root = new NSView ();
+			NSImageView imageView = new NSImageView () {
+				Image = image
+			};
+			root.AddSubview (imageView);
+
+			NSTextView text = new NSTextView () {
+				Value = "Hello World",
+			};
+			root.AddSubview (text);
+			return root;
+		}
+
+		static YogaNode CalculateLayout (CGSize imageSize)
+		{
+			var rootNode = new YogaNode () {
+				Width = 400,
+				Height = 120,
+				FlexDirection = YogaFlexDirection.Row
+			};
+			rootNode.SetPadding (YogaEdge.All, 20);
+
+			var imageNode = new YogaNode () {
+				Width = (float)imageSize.Width,
+			};
+			imageNode.SetMargin (YogaEdge.End, 20);
+
+			var textNode = new YogaNode () {
+				AlignSelf = YogaAlign.Center,
+				FlexGrow = 1,
+			};
+			rootNode.Insert (0, imageNode);
+			rootNode.Insert (1, textNode);
+			rootNode.CalculateLayout ();
+
+			return rootNode;
+		}
+	}
+}