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/CustomBuildAction.targets

@@ -0,0 +1,11 @@
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+	<PropertyGroup>
+	    <CompileDependsOn>CopyInNativeLib;$(CompileDependsOn)</CompileDependsOn>
+	</PropertyGroup>
+
+	<!-- The # and , in this path does not play nice with the binding project logic, so make a copy -->
+	<Target Name="CopyInNativeLib" Inputs="..\..\..\buck-out\gen\yoga#default,static\libyoga.a" Outputs="$(ProjectDir)libyoga.a">
+	    <Copy SourceFiles="..\..\..\buck-out\gen\yoga#default,static\libyoga.a" DestinationFiles="$(ProjectDir)/libyoga.a" />
+	</Target>
+</Project>