Add YGLayoutGetMargin

Summary:
Fix #326. I'll open another PR once this one gets accepted to add support for `YGLayoutGetBorder` 👌
Closes https://github.com/facebook/yoga/pull/335

Reviewed By: gkassabli

Differential Revision: D4409399

Pulled By: emilsjolander

fbshipit-source-id: 8153f6701cab60b55a485f6d2e0b9f7767481090

csharp/tests/Facebook.Yoga/YogaNodeTest.cs

@@ -434,6 +434,23 @@ namespace Facebook.Yoga
             });
         }
 
+        [Test]
+        public void TestLayoutMargin() {
+          YogaNode node = new YogaNode();
+          node.Width = 100;
+          node.Height = 100;
+          node.MarginStart = 1;
+          node.MarginEnd = 2;
+          node.MarginTop = 3;
+          node.MarginBottom = 4;
+          node.CalculateLayout();
+
+          Assert.AreEqual(1, node.LayoutMarginLeft);
+          Assert.AreEqual(2, node.LayoutMarginRight);
+          Assert.AreEqual(3, node.LayoutMarginTop);
+          Assert.AreEqual(4, node.LayoutMarginBottom);
+        }
+
         [Test]
         public void TestLayoutPadding() {
           YogaNode node = new YogaNode();