Add YogaKit Shared and iOS version

Summary:
Using shared code for reuse in other platforms based on iOS native implementation.

Adds YogaKit sample.

Adds YogaKit tests (same as objc).

```
YogaKitTest : 80 ms
Facebook.YogaKit.iOS.Tests.exe : 81 ms
Tests run: 11 Passed: 8 Inconclusive: 0 Failed: 3 Ignored: 1
```

Since we don't have extension properties we need to go with a extension method to get access to the YogaLayout .

I m also not sure this is leak free yet, would love some help with testing and feedback about view/node lifecycle
Closes https://github.com/facebook/yoga/pull/336

Reviewed By: splhack

Differential Revision: D4415027

Pulled By: emilsjolander

fbshipit-source-id: c88328212426c3200e6f0c48cda594cd2c432065

csharp/iOS/Facebook.YogaKit.iOS.Tests/Info.plist

@@ -0,0 +1,36 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"[]>
+<plist version="1.0">
+    <dict>
+        <key>CFBundleName</key>
+        <string>Facebook.YogaKit.iOS.Tests</string>
+        <key>CFBundleIdentifier</key>
+        <string>com.xamarin.facebook-yogakit-ios-tests</string>
+        <key>CFBundleShortVersionString</key>
+        <string>1.0</string>
+        <key>CFBundleVersion</key>
+        <string>1.0</string>
+        <key>LSRequiresIPhoneOS</key>
+        <true />
+        <key>MinimumOSVersion</key>
+        <string>10.2</string>
+        <key>UIDeviceFamily</key>
+        <array>
+            <integer>1</integer>
+            <integer>2</integer>
+        </array>
+        <key>UISupportedInterfaceOrientations</key>
+        <array>
+            <string>UIInterfaceOrientationPortrait</string>
+            <string>UIInterfaceOrientationLandscapeLeft</string>
+            <string>UIInterfaceOrientationLandscapeRight</string>
+        </array>
+        <key>UILaunchStoryboardName</key>
+        <string>LaunchScreen</string>
+        <key>NSAppTransportSecurity</key>
+        <dict>
+            <key>NSAllowsArbitraryLoads</key>
+            <true />
+        </dict>
+    </dict>
+</plist>