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

yoga/YGMacros.h

@@ -40,3 +40,19 @@
     YG_ABORT();                            \
   }
 #endif
+
+#ifndef YG_ENUM_BEGIN
+#ifndef NS_ENUM
+#define YG_ENUM_BEGIN(name) enum name
+#else
+#define YG_ENUM_BEGIN(name) NS_ENUM(NSInteger, name)
+#endif
+#endif
+
+#ifndef YG_ENUM_END
+#ifndef NS_ENUM
+#define YG_ENUM_END(name) name
+#else
+#define YG_ENUM_END(name)
+#endif
+#endif