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

javascript/sources/nbind.cc

@@ -65,6 +65,7 @@ NBIND_CLASS(Node)
 
     method(setMargin);
     method(setMarginPercent);
+    method(setMarginAuto);
 
     method(setOverflow);
     method(setDisplay);
@@ -77,8 +78,10 @@ NBIND_CLASS(Node)
 
     method(setWidth);
     method(setWidthPercent);
+    method(setWidthAuto);
     method(setHeight);
     method(setHeightPercent);
+    method(setHeightAuto);
 
     method(setMinWidth);
     method(setMinWidthPercent);