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/YGLayout+Private.h

@@ -0,0 +1,16 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#import "YGLayout.h"
+
+@interface YGLayout (Private)
+
+- (instancetype)initWithView:(UIView *)view;
+
+@end