always display module id's, not filepaths

I don't expect this pull request to be accepted.  It introduces a new dependency (underscore), doesn't really follow precedent coding style, and I just noticed I failed to remove a couple of useless and misleading comments. :p So please don't accept it!

I would like to get some feedback on the basic idea of the changes, though!  I opened #41, explored the code a bit, then made these changes.  It works well for me, and if @pahen wants to merge them in, I'll submit a new cleaned-up PR.

#41 explains somewhat what I was looking for, but essentially I was getting a log of filepaths my dependency tree output (both text and image), and I wanted to see module id's instead.  I was also getting duplicates, such as `js/lib/backbone.min.js` and also `backbone`.  This change merges those two into just `backbone`.

Thoughts?