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

gentest/gentest.js

@@ -27,6 +27,12 @@ window.onload = function() {
       document.body.children[0],
       document.body.children[1],
       document.body.children[2]);
+
+  printTest(
+      new JavascriptEmitter(),
+      document.body.children[0],
+      document.body.children[1],
+      document.body.children[2]);
 }
 
 function assert(condition, message) {