Improved abslute path resolution / Options to choose minified ressources

Hi Alessandro, 

- I tried to improve the function that's guessing absolute file path from a file URL. I think it could be more accurate to deal with relative path and other exceptions. 

- I added two options in the admin area. They give the user more control over the type of ressource that should be minified. I did it because moving CSS files to another location breaks every background images as they are nearly always included with relative path (of the theme). I think it's a problem for many users but I didn't figured out an easy way to solve (moving images ? replacing CSS background URLs ?). 
The users can now enjoy script minification and concatenation even if their background images are not made of absolute URLs.