Browserify

This starts to move dependencies into package.json and package them with
browserify. Essentially:

* This would let us more easily version dependencies
* And use specific namespace'd requires if necessary (may make the _
  issue easier to handle)
* Would make it more straightforward to test and develop more parts of
  iD outside of the core.

On the downside

* This makes inline development of dependencies more complex
* And it adds another build step & build tool. Though, we already
  require uglify, so 'development' already requires nodejs.