Allow options attributes

Just a thought ... to improve the usability of long options, I added "title" and "css" (Beautytips) ... end users said thanks.

$boxhtml .= "<option class=\"beautytips\" title=\"". $name ."\" value=\"". $value ."\">". $name ."</option>\n";

In:

function _multiselect_html_for_unselected_box_options($unselected_options) {
  $boxhtml = '';
  foreach ($unselected_options as $value => $name) {
    $boxhtml .= "<option class=\"beautytips\" title=\"". $name ."\" value=\"". $value ."\">". $name ."</option>\n";
  }
  return $boxhtml;
}

Thanks for this great module!