Improve JS bindings

Summary:
- Fix a few things related to how npm should publish the package (sources were in the gitignore file, so I think it would have break the build during install - I fixed this by adding a npmignore that should override the gitignore rules)

- The enumerations values are now generated directly from `enums.py`

- I added percent unit support (#258) to the bindings (`.setWidthPercent` is currently exposed, but I've also added a very very little parsing to also support `.setWidth("100%")` and `.setWidth(.getWidth())`), added the missing tests, and fixed Travis.
Closes https://github.com/facebook/yoga/pull/314

Reviewed By: mikearmstrong001

Differential Revision: D4377198

Pulled By: emilsjolander

fbshipit-source-id: 774dfafd416f5421f3be59a1d181eb7056487abe

javascript/sources/Value.hh

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <yoga/Yoga.h>
+
+struct Value
+{
+    static Value fromYGValue(YGValue const & ygValue)
+    {
+        return Value(static_cast<int>(ygValue.unit), ygValue.value);
+    }
+
+    int unit;
+    double value;
+
+    Value(void)
+    : unit(YGUnitUndefined)
+    , value(0.0)
+    {
+    }
+
+    Value(int unit, double value)
+    : unit(unit)
+    , value(value)
+    {
+    }
+
+    void toJS(nbind::cbOutput expose) const
+    {
+        expose(unit, value);
+    }
+};