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-cs.js

@@ -17,6 +17,9 @@ function toCsUnitValue(value) {
   if (value.indexOf('%') >= 0){
     methodName = '.Percent()';
   }
+  if(value.indexOf('Auto') >= 0){
+    return 'YogaValue.Auto()';
+  }
   return toValueCs(value) + methodName;
 }
 
@@ -130,6 +133,8 @@ CSEmitter.prototype = Object.create(Emitter.prototype, {
 
   YGUndefined:{value:'YogaConstants.Undefined'},
 
+  YGAuto:{value:'YogaConstants.Auto'},
+
   YGDisplayFlex:{value:'YogaDisplay.Flex'},
   YGDisplayNone:{value:'YogaDisplay.None'},