def cd(dir)
      dir = dir.location if dir.is_a? FolderObject
      @current_dir = File.expand_path(dir,@current_dir)
      self
    end
def cd(dir)
      dir = dir.location if dir.is_a? FolderObject
      @current_dir = File.expand_path(dir,@current_dir)
      self
    end
Changes to the given directory. Will accept an AirVideo::FolderObject or a string.
 Returns the AirVideo::Client instance, so you can string commands:

     AirVideo::Client.new('127.0.0.1').ls[0].cd.ls

 NB. This will *not* check to see if the folder actually exists!