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

docs/_docs/api/javascript.md

@@ -0,0 +1,53 @@
+---
+docid: javascript
+title: Javascript
+layout: docs
+permalink: /docs/api/javascript/
+---
+
+The `yoga-layout` module offers two different implementations of Yoga. The first one is a native Node module, and the second one is an asm.js fallback that can be used when native compilation isn't an option - such as web browsers.
+
+> Because this module is compiled from a C++ codebase, the function prototypes below will use the C++-syntax. Nevertheless, the corresponding methods all exist on the JS side, with the same arguments (`Node *` being a node object).
+
+### Lifecycle
+
+Create a node via `Yoga.Node.create()`, and destroy it by using its `free()` or `freeRecursive()` method. Note that unlike most objects in your Javascript applications, your nodes will not get automatically garbage collected, which means that it is especially important you keep track of them so you can free them when you no longer need them.
+
+<script src="http://gist-it.appspot.com/facebook/yoga/raw/master/javascript/sources/Node.hh?slice=14:17"></script>
+
+### Children
+
+The following methods help manage the children of a node.
+
+<script src="http://gist-it.appspot.com/facebook/yoga/raw/master/javascript/sources/Node.hh?slice=114:126"></script>
+
+### Style getters & setters
+
+The large part of Yoga's API consists of properties, setters, and getters for styles. These all follow the same general structure. Bellow are the function and enums used to control the various styles. For an in depth guide to how each style works see the getting started guide.
+
+<script src="http://gist-it.appspot.com/facebook/yoga/raw/master/javascript/sources/Node.hh?slice=41:77"></script>
+
+<script src="http://gist-it.appspot.com/facebook/yoga/raw/master/javascript/sources/Node.hh?slice=79:112"></script>
+
+### Layout results
+
+Once you have set up a tree of nodes with styles you will want to get the result of a layout calculation. Call `calculateLayout()` perform layout calculation. Once this function returns the results of the layout calculation is stored on each node. Traverse the tree and retrieve the values from each node.
+
+<script src="http://gist-it.appspot.com/facebook/yoga/raw/master/javascript/sources/Node.hh?slice=142:157"></script>
+
+### Custom measurements
+
+Certain nodes need to ability to measure themselves, the most common example is nodes which represent text. Text has an intrinsic size and requires measuring itself to determine that size. This is not something Yoga can do as it requires relying on the host system's text rendering engine.
+
+- Call `markDirty()` if a node with a custom text measurement function needs to be re-measured during the next layout pass.
+- Note that in the prototype below, `Size` is an object that contain both `width` and `height` properties.
+
+> A measure function can only be attached to a leaf node in the hierarchy.
+
+<script src="http://gist-it.appspot.com/facebook/yoga/raw/master/javascript/sources/Node.hh?slice=128:131"></script>
+
+### Experiments
+
+Yoga has the concept of experiments. An experiment is a feature which is not yet stable. To enable a feature use the following functions. Once a feature has been tested and is ready to be released as a stable API we will remove its feature flag.
+
+<script src="http://gist-it.appspot.com/facebook/yoga/raw/master/javascript/sources/global.hh?slice=3:4"></script>