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/Node.hh

@@ -65,6 +65,7 @@ class Node {
 
     void setMargin(int edge, double margin);
     void setMarginPercent(int edge, double margin);
+    void setMarginAuto(int edge);
 
     void setOverflow(int overflow);
     void setDisplay(int display);
@@ -72,13 +73,16 @@ class Node {
     void setFlex(double flex);
     void setFlexBasis(double flexBasis);
     void setFlexBasisPercent(double flexBasis);
+    void setFlexBasisAuto();
     void setFlexGrow(double flexGrow);
     void setFlexShrink(double flexShrink);
 
     void setWidth(double width);
     void setWidthPercent(double width);
+    void setWidthAuto();
     void setHeight(double height);
     void setHeightPercent(double height);
+    void setHeightAuto();
 
     void setMinWidth(double minWidth);
     void setMinWidthPercent(double minWidth);