added support for a 'url' parameter in the 'roots' section. Used  to specify specific url to prefix to the file name, for displaying/downloading. Useful for file stored outside the document root
added support for relative url paramters
updated README.md to reflect the new url parameter
reverted file permissions
Merge pull request #29 from rjbijl/master

Added support for an optional 'url' parameter in the 'roots' section