Show message and don't start if javascript files have not been built

fixes #1782
- added builtFilesExist function to check for files during startup.
  If built files do not exist Ghost startup is stopped and a link
  to the documentation is displayed.
- exported a scriptFiles object from server/helpers.
- added a builtScriptPath to the paths module.
- removed "js-msg" about missing javascript from the UI.

ghost/admin/init.js

@@ -49,9 +49,6 @@
     };
 
     Ghost.init = function () {
-        // remove the temporary message which appears
-        $('.js-msg').remove();
-
         Ghost.router = new Ghost.Router();
 
         // This is needed so Backbone recognizes elements already rendered server side