Forcing relative path

If style rules include relative paths in its URLs and we are using a "forceRelative" option, we should really only apply the file name to the forceRelative path.

Example:
forceRelative: "/app/images/"
background: url('../some/folder/bg.jpg');

should really compile to:
background: url('/app/images/bg.jpg')

and not
background: url('/app/images/../some/folder/bg.jpg')