Add debug path support

One of my favorite features from JHtml is the ability to load a different file if the site is in debug mode, which is useful if you default to loading compressed CSS/JS files and need to debug those.  Borrowing a little logic from JHtml::includeRelativeFiles(), I've added the code to do this in FOF.
Merge pull request #2 from mbabker/patch-1

Add debug path support. Thank you, Michael!