def on(cls, &block)
      @current_screen = @current_page = cls.new
      waiting_for  = "#{cls} to be active"
      wait_until(10, waiting_for) { @current_screen.active? } if @current_screen.respond_to?(:active?)
      block.call @current_screen if block
      @current_screen
    end
def on(cls, &block)
      @current_screen = @current_page = cls.new
      waiting_for  = "#{cls} to be active"
      wait_until(10, waiting_for) { @current_screen.active? } if @current_screen.respond_to?(:active?)
      block.call @current_screen if block
      @current_screen
    end
create a new screen given a class name