def clean(&block)
      @target.files.each do |object|
        unless @source.files.include? object.key
          block.call(object)
          object.destroy
        end
      end
    end
def clean(&block)
      @target.files.each do |object|
        unless @source.files.include? object.key
          block.call(object)
          object.destroy
        end
      end
    end
Removes files from target that don't exist on the source.