added test for external dependencies

Added a test that uses config.js to specify where external dependencies are. Directory structure:

* src
  * index.js - depends on `extra`
* deps
  * extra
    * extra.js
    * lib
      * helper.js
* config.js - `{paths: { extra: "deps/extra/extra" } }`

I relatively new to `gulp`, `requirejs` and CoffeeScript, so this test may need to be modified. The intent of the test is to emulate a relatively complex `bower` + `requirejs` setup.