Use browserify to make js code more modular.

* Moved js library code to `js/lib`.
* `js/stmd.js` is now generated from these files using browserify.
* Factored out `html5-entities.js` and `from-code-point.js` from main
  js parsing code (which is now `index.js`).
* Moved `js/markdown` to `js/bin`.
Change from-code-point.js so it doesn't alter String prototype.