Move YGStyle to seperate file and add constructors

Summary: Moved YGStyle to separate file and added default constructor to it. This diff also gets rid off of the default values, which were earlier declared in Yoga-interna.h and instead uses default constructor. This diff also addresses https://github.com/facebook/yoga/pull/700

Reviewed By: emilsjolander

Differential Revision: D7016575

fbshipit-source-id: eb28df0ffb4cc813b23edaff80d7d4ebc56ce6af

tests/YGStyleTest.cpp

@@ -7,6 +7,7 @@
 
 #include <gtest/gtest.h>
 #include <yoga/YGNode.h>
+#include <iostream>
 
 TEST(YogaTest, copy_style_same) {
   const YGNodeRef node0 = YGNodeNew();