Update wp-less.php

- path_join vs. trailingslashit
- wp_mkdir_p include the file_exists check > http://core.trac.wordpress.org/browser/tags/3.5.1/wp-includes/functions.php#L1335

Prost ;)
Merge pull request #36 from sergejmueller/patch-1

removed doubled file_exists() check and exchanged path composition with easier path_join() function