Adds stitch-rb support to rake file

Fixes issue #890

Assumptions:
* modules will be in {source_dir}/javascripts/modules
* Stitch writes to a temporary (un-minified)
  {source_dir}/javascripts/octopress.js (added to .gitignore)
* libs not part of a module are still in {source_dir}/javascripts/group
  and prepended to octopress.min.js
* CoffeeScript is supported only if added to the Gemfile
  (CS modules will fail with a misleading error if `gem 'coffee-script'`
  is not added to Gemfile)