dynamic file caching pseudo cdn
- cdn root path : http://cdn.com/
- cdn example url : http://cdn.com/path/to/resource.css?d=12345
- maps the uri : /path/to/resource.css?d=12345
- to the origin : http://yoursite.com/path/to/resource.css?d=12345
- caches file to : ./cache/[base64-encoded-uri].css
- returns local cached copy or issues 304 not modified