def add_class(selector, *class_names)
      select(selector).each do |element|
        class_list = ClassList.new(element)
        class_list.add class_names
      end
    end
def add_class(selector, *class_names)
      select(selector).each do |element|
        class_list = ClassList.new(element)
        class_list.add class_names
      end
    end
Adds the given class names to the selected elements.

 @param [String] selector a CSS or XPath string describing the elements to
   transform
 @param [String] class_names a CSS class name that should be added
 @example
   add_class('a#home', 'selected')
   find('a#home').add_class('selected')