Start work on JavaScript API client.

- lib/link.js is so far an incomplete implementation of a DSLink. It's based off of link.dart, as well as the specification.
- lib/util.js is utilities that I use across the library via the un
derscore variable. I'm aware that Underscore has everything in here
, but util.js is extremely small.
- index.js just multiplexes all the different sublibraries together and exposes them to module.exports.
- Added mocha/better-assert to the package.json as developer dependencies. These will be used later for unit testing.