return relative paths of n levels, including basename.
        eg: File('a/b/c/d').n_parent_paths(3) == 'b/c/d'

def n_relative_paths(self, n):
        """
        return relative paths of n levels, including basename.
        eg: File('a/b/c/d').n_parent_paths(3) == 'b/c/d'
        """
        rv = os.path.join(self.n_parent_paths(n-1), self.basename)
        return rv.replace('\\', '/')