Add support for Drush 4

drush_ctex_bonus works great but it depends on 1 function in the latest drush that is not always available in the latest packages

I'd suggest we'd add something like this

if (!function_exists('drush_trim_path')) {

function drush_trim_path($path, $os = NULL) {
  if (drush_is_windows($os)) {
    return rtrim($path, '/\\');
  }
  else {
    return rtrim($path, '/');
  }
}
  
}

by adding this to my code, drush 4.6 is perfectly happy. We should perhaps think about adding it to this module