Adds Javascript Support

Summary:
- As mentioned in the title, this PR adds Javascript support to Yoga. Two different builds are included in this PR thanks to [nbind](https://github.com/charto/nbind), which conveniently allow to target both Node.js' native addons and browser environments via asmjs with approximately the same codebase. That should solve #215.

- All tests successfully pass on both codepaths. You can run `yarn test:all` inside the `javascript` directory to test it.

- Because of a bug in nbind, the [following PR](https://github.com/charto/nbind/pull/57) needs to be merged and a new version released before this one can be safely merged as well.

- I had to use `double` types instead of `float` in the C++ bindings because of an Emscripten [bug](https://github.com/kripken/emscripten/issues/3592) where symbols aren't correctly exported when using floats.

- There's some tweaks to do before this PR is 100% ready to merge, but I wanted to have your opinion first. What do you think of this?

 ---

To do:

- [x] Ensure th
Closes https://github.com/facebook/yoga/pull/304

Reviewed By: mikearmstrong001

Differential Revision: D4375187

Pulled By: emilsjolander

fbshipit-source-id: 47248558a9506b7c512b5ef281cd12fe1a60cab7

javascript/tests/tools.js

@@ -0,0 +1,60 @@
+/**
+ * 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.
+ */
+
+var target = typeof global !== "undefined" ? global : window;
+
+target.getMeasureCounter = function (Yoga, cb, staticWidth, staticHeight) {
+
+    var counter = 0;
+
+    return {
+
+        inc: function (width, widthMode, height, heightMode) {
+
+            counter += 1;
+
+            return cb ? cb(width, widthMode, height, heightMode) : { width: staticWidth, height: staticHeight };
+
+        },
+
+        get: function () {
+
+            return counter;
+
+        }
+
+    };
+
+}
+
+target.getMeasureCounterMax = function (Yoga) {
+
+    return getMeasureCounter(Yoga, function (width, widthMode, height, heightMode) {
+
+        var measuredWidth = widthMode === Yoga.MEASURE_MODE_UNDEFINED ? 10 : width;
+        var measuredHeight = heightMode === Yoga.MEASURE_MODE_UNDEFINED ? 10 : height;
+
+        return { width: measuredWidth, height: measuredHeight };
+
+    });
+
+};
+
+target.getMeasureCounterMin = function (Yoga) {
+
+    return getMeasureCounter(Yoga, function (width, widthMode, height, heightMode) {
+
+        var measuredWidth = widthMode === Yoga.MEASURE_MODE_UNDEFINED || (widthMode == Yoga.MEASURE_MODE_AT_MOST && width > 10) ? 10 : width;
+        var measuredHeight = heightMode === Yoga.MEASURE_MODE_UNDEFINED || (heightMode == Yoga.MEASURE_MODE_AT_MOST && height > 10) ? 10 : height;
+
+        return { width: measuredWidth, height: measuredHeight };
+
+    });
+
+};