Improve JS bindings

Summary:
- Fix a few things related to how npm should publish the package (sources were in the gitignore file, so I think it would have break the build during install - I fixed this by adding a npmignore that should override the gitignore rules)

- The enumerations values are now generated directly from `enums.py`

- I added percent unit support (#258) to the bindings (`.setWidthPercent` is currently exposed, but I've also added a very very little parsing to also support `.setWidth("100%")` and `.setWidth(.getWidth())`), added the missing tests, and fixed Travis.
Closes https://github.com/facebook/yoga/pull/314

Reviewed By: mikearmstrong001

Differential Revision: D4377198

Pulled By: emilsjolander

fbshipit-source-id: 774dfafd416f5421f3be59a1d181eb7056487abe

javascript/package.json

@@ -28,7 +28,9 @@
 
         "bench": "node tests/run-bench $(find tests/Benchmarks -name '*.js')",
 
-        "install": "npm -- run autogypi && npm -- run build:node"
+        "install": "npm -- run autogypi && npm -- run build:node",
+
+        "prepare": "npm -- run build:browser"
 
     },