Feature auto margin

Summary:
Even so I know there are some opinions against ```margin: 0 auto``` it's still part of the spec: https://www.w3.org/TR/css-flexbox-1/#auto-margins and pretty usefull if you have to position via ```justify-content```.

This PR adds an implementation for that.

It adds an additonal ```YGUnitAuto``` and margins got ```YGNodeStyleSetMarginAuto``` functions as well.
Closes https://github.com/facebook/yoga/pull/357

Reviewed By: astreet

Differential Revision: D4501142

Pulled By: emilsjolander

fbshipit-source-id: 86519f8632496f46e78a7c9dbc5b21e212e3e0c7

gentest/gentest-javascript.js

@@ -14,6 +14,7 @@ var JavascriptEmitter = function() {
 function toValueJavascript(value) {
   if (value.match(/^[0-9.e+-]+px$/i)) return parseFloat(value);
   if (value.match(/^[0-9.e+-]+%/i)) return JSON.stringify(value);
+  if (value == 'Yoga.AUTO') return '"auto"';
   return value;
 }
 
@@ -122,6 +123,8 @@ JavascriptEmitter.prototype = Object.create(Emitter.prototype, {
   YGPositionTypeAbsolute:{value:'Yoga.POSITION_TYPE_ABSOLUTE'},
   YGPositionTypeRelative:{value:'Yoga.POSITION_TYPE_RELATIVE'},
 
+  YGAuto:{value:'Yoga.AUTO'},
+
   YGWrapNoWrap:{value:'Yoga.WRAP_NO_WRAP'},
   YGWrapWrap:{value:'Yoga.WRAP_WRAP'},