Changes for nuget and CI support

Summary:
For now i setup the CI using VSTS (Visual Studio Team Services) , i m using my own account, but i want to move this to a Travis setup so it would be easy ported to Facebook own build system. We can also take a look on how buck can help with this.

Here's a overview of my dashboard for the build and release process on VSTS:

<img width="1084" alt="screen shot 2017-02-16 at 23 52 07" src="https://cloud.githubusercontent.com/assets/1235097/23046869/ff4dcada-f4a3-11e6-8d09-fb8974fb58d4.png">

The all process is pretty simple, there's a few tricks here and there, but the basic steps are:

<img width="1036" alt="builds" src="https://cloud.githubusercontent.com/assets/1235097/23046874/0cb95fc2-f4a4-11e6-8fd8-3048be7ffd9d.png">

These run for every commit and publish the artifacts, then the release lane takes that artifacts and builds a nuget, after that (and it's not implemented yet) we run UI Tests on devices Android , iOS for the nuget created and run tests on UWP and .net45 on both windows and OS X so
Closes https://github.com/facebook/yoga/pull/408

Reviewed By: emilsjolander

Differential Revision: D4576542

Pulled By: splhack

fbshipit-source-id: 66e0e4c1cb3721ed7aa7118e4e226b6ec575fea8

csharp/build-native.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+if buck --version >/dev/null 2>&1; then true; else
+    echo "Building Buck!"
+    mkdir lib
+    cd lib
+    git clone https://github.com/facebook/buck.git --depth 1
+    cd buck
+    ant
+    cd ..
+    cd ..
+fi
+buck build //:yoga
+buck build //csharp:yoganet-ios
+buck build //csharp:yoganet-macosx
+buck build //csharp:yoganet#android-x86,shared
+buck build //csharp:yoganet#android-armv7,shared