Xamarin iOS Support

Summary:
Since there's a couple of changes after last month pr of #280 , here's a new pr for Xamarin support #276 , the idea is to add Xamarin iOS, Mac and Android support.

- Add Xamarin iOS support for Yoga, by providing a wrapper for the native lib.
- Add Xamarin iOS Unit testing project
~~- Adds a YogaKit shared implementation in c#.~~
~~- Adds a YogaKIt iOS implementation.~~
~~- Adds Yoga/YogaKit iOS sample application~~

Facebook.Yoga.iOS.Tests.exe : 169 ms
Tests run: 114 Passed: 114 Inconclusive: 0 Failed: 0 Ignored: 0

![simulator screen shot 6 jan 2017 02 05 15](https://cloud.githubusercontent.com/assets/1235097/21705017/f851e9d8-d3b4-11e6-9c44-646dab559643.png)
Closes https://github.com/facebook/yoga/pull/324

Reviewed By: emilsjolander

Differential Revision: D4401408

Pulled By: splhack

fbshipit-source-id: ecdae7967060361bef2bc25a5ef759cb9a957322

csharp/tests/Facebook.Yoga/Facebook.Yoga.Shared.Tests.projitems

@@ -22,7 +22,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)YGMinMaxDimensionTest.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)YGPaddingTest.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)YGRoundingTest.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)YogaNodeCreateTest.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)YogaNodeSpacingTest.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)YogaNodeTest.cs" />
   </ItemGroup>
-</Project>
+</Project>