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/Facebook.Yoga/YGMeasureCacheTest.js

@@ -0,0 +1,35 @@
+/**
+ * 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 Yoga = Yoga || require("../../sources/entry-" + process.env.TEST_ENTRY);
+
+it("measure_once_single_flexible_child", function () {
+  var root = Yoga.Node.create();
+  root.setFlexDirection(Yoga.FLEX_DIRECTION_ROW);
+  root.setAlignItems(Yoga.ALIGN_FLEX_START);
+  root.setWidth(100);
+  root.setHeight(100);
+
+  var measureCounter = getMeasureCounterMax(Yoga);
+
+  var root_child0 = Yoga.Node.create();
+  root_child0.setMeasureFunc(measureCounter.inc);
+  root_child0.setFlexGrow(1);
+  root.insertChild(root_child0, 0);
+
+  root.calculateLayout(Yoga.UNDEFINED, Yoga.UNDEFINED, Yoga.DIRECTION_LTR);
+
+  console.assert(1 === measureCounter.get(), "1 === measureCounter.get()");
+
+  if (typeof root !== "undefined")
+    root.freeRecursive();
+
+  (typeof gc !== "undefined") && gc();
+  console.assert(0 === Yoga.getInstanceCount(), "0 === Yoga.getInstanceCount() (" + Yoga.getInstanceCount() + ")");
+});