Add YGLayoutGetBorder, counterpart of YGLayoutGetPadding

Summary:
Followup of #335, fix #326. This commit add the `YGLayoutGetBorder(node, edge)` function, which correctly takes RTL/LTR into account when resolving `EDGE_START` & `EDGE_END`.
Closes https://github.com/facebook/yoga/pull/344

Reviewed By: dshahidehpour

Differential Revision: D4459950

Pulled By: emilsjolander

fbshipit-source-id: b57eb7a5b1c181a364913c3200a3794a2b7b31a6

javascript/sources/nbind.cc

@@ -155,5 +155,6 @@ NBIND_CLASS(Node)
     method(getComputedLayout);
 
     method(getComputedMargin);
+    method(getComputedBorder);
     method(getComputedPadding);
 }