Change require_tree to use project paths

[samsonasu]

sprockets require_tree uses paths that are strictly relative to the file, which creates inconsistent behavior, i.e. for a directory structure like this:

• app
  • controllers
    • some_controller.js
  • app.js
• config
  • high_five
    • app-common.js

// This works:
//= require app/app.js
// This should work but doesn't: 
//= require_tree app/controllers
//  So we have to write the require_tree as such: 
//= require_tree ../../app/controllers

This might require either a patch to sprockets or a new directive processor, I'm not sure if the sprockets team would be open