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

java/com/facebook/yoga/YogaUnit.java

@@ -15,7 +15,8 @@ import com.facebook.proguard.annotations.DoNotStrip;
 public enum YogaUnit {
   UNDEFINED(0),
   PIXEL(1),
-  PERCENT(2);
+  PERCENT(2),
+  AUTO(3);
 
   private int mIntValue;
 
@@ -32,6 +33,7 @@ public enum YogaUnit {
       case 0: return UNDEFINED;
       case 1: return PIXEL;
       case 2: return PERCENT;
+      case 3: return AUTO;
       default: throw new IllegalArgumentException("Unknown enum value: " + value);
     }
   }