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/iOS/Facebook.Yoga.iOS.Tests/LaunchScreen.storyboard

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="9532" systemVersion="15D21" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" launchScreen="YES" useTraitCollections="YES" initialViewController="01J-lp-oVM">
+    <dependencies>
+        <deployment identifier="iOS" />
+        <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="9530" />
+    </dependencies>
+    <scenes>
+        <!--View Controller-->
+        <scene sceneID="EHf-IW-A2E">
+            <objects>
+                <viewController id="01J-lp-oVM" sceneMemberID="viewController">
+                    <layoutGuides>
+                        <viewControllerLayoutGuide type="top" id="Llm-lL-Icb" />
+                        <viewControllerLayoutGuide type="bottom" id="xb3-aO-Qok" />
+                    </layoutGuides>
+                    <view key="view" contentMode="scaleToFill" id="Ze5-6b-2t3">
+                        <rect key="frame" x="0.0" y="0.0" width="600" height="600" />
+                        <autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES" />
+                        <color key="backgroundColor" white="1" alpha="1" colorSpace="custom" customColorSpace="calibratedWhite" />
+                    </view>
+                </viewController>
+                <placeholder placeholderIdentifier="IBFirstResponder" id="iYj-Kq-Ea1" userLabel="First Responder" sceneMemberID="firstResponder" />
+            </objects>
+            <point key="canvasLocation" x="53" y="375" />
+        </scene>
+    </scenes>
+</document>