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

.travis.yml

@@ -17,14 +17,26 @@ before_install:
   - brew cask install java
   - brew outdated xctool || brew upgrade xctool
   - brew install mono
+
   - export JAVA_HOME=$(/usr/libexec/java_home -v 1.8)
   - export PATH=$JAVA_HOME/bin:$PATH
 
+install:
+  - cd javascript
+  - npm install
+  - cd $TRAVIS_BUILD_DIR
+
 script:
   - buck test //:yoga
   - buck test //java:java
   - buck test //YogaKit:YogaKitTests --config cxx.default_platform=iphonesimulator-x86_64 --config cxx.cflags=-DTRAVIS_CI
   - sh csharp/tests/Facebook.Yoga/test_macos.sh
+
+  - cd javascript
+  - npm run test:all
+  - npm run bench
+  - cd $TRAVIS_BUILD_DIR
+
   - buck run //benchmark:benchmark
   - git checkout HEAD^
   - buck run //benchmark:benchmark