Improved the objective-c and swift api

Summary:
Compared to what was planned, I added the `overflow` value which seemed missing. I had to modify the implementation a bit for all values which are backed by a `YGValue`, but we should probably enable the pixel dimensions in Objective-C and Swift somehow later.
Closes https://github.com/facebook/yoga/pull/322

Reviewed By: cosmin1123

Differential Revision: D4391434

Pulled By: emilsjolander

fbshipit-source-id: e33f6f7b2bbaad29553100b7a5bb424496372110

YogaKit/YogaKitSample/YogaKitSample/SwiftViewController.swift

@@ -0,0 +1,40 @@
+/**
+ * Copyright 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the license found in the
+ * LICENSE-examples file in the root directory of this source tree.
+ */
+
+import UIKit
+
+class SwiftViewController: UIViewController {
+  override func viewDidLoad() {
+    let root = self.view!
+    root.backgroundColor = .red
+    root.yoga.isEnabled = true
+    root.yoga.width = self.view.bounds.size.width
+    root.yoga.height = self.view.bounds.size.height
+    root.yoga.alignItems = .center
+    root.yoga.justifyContent = .center
+
+    let child1 = UIView()
+    child1.backgroundColor = .blue
+    child1.yoga.isEnabled = true
+    child1.yoga.width = 100
+    child1.yoga.height = 10
+
+    let child2 = UIView()
+    child2.backgroundColor = .green
+    child2.frame = CGRect(x: 0, y: 0, width: 200, height: 100)
+
+    let child3 = UIView()
+    child3.backgroundColor = .yellow
+    child3.frame = CGRect(x: 0, y: 0, width: 100, height: 100)
+
+    child2.addSubview(child3)
+    root.addSubview(child1)
+    root.addSubview(child2)
+    root.yoga.applyLayout()
+  }
+}