ignore js/css/img hosted on a remote server

- cacheBust should ignore files hosted on a CDN as these files typically
  include version strings to identify code changes

- including a hash value in the querystring at best causes a cache miss
  in the browser, causing the browser to redownload already cached
  version of CDN hosted libraries like jQuery, Bootstrap, or AngularJS;
  at worst, the remote server might alter the response based on a the
  querystring parameter

- the current cacheBust implementation does not accurately hash remote
  URLs, as it would need to download the URL to a local file first