Refactoring of model referencing + path finding

Close #2: node-webkit can be in ~/Application

Changes:
* models.js is no more loadind from brwser context but node.js. Slightly
  cleaner, and prepare the separation gui/npm
* path_finder.js knows some paths and try to find the best node-webkit binary in
  the known paths. TODO: Should be configurable.
* use path.join and path-extra for cross-platform paths (mostly path_finder)