def javascript_bundle(key, files = nil)
      javascript_bundles[key] = JavascriptBundle.new(self, key.to_s, files)
    end
def javascript_bundle(key, files = nil)
      javascript_bundles[key] = JavascriptBundle.new(self, key.to_s, files)
    end
Set a Javascript bundle
    javascript_bundle(:all, %w(jquery lightbox))
 @param [Symbol,String] key The bundle name
 @param [Array(String)] files The list of filenames, without extension,
   assumed to be in the public directory, under 'javascripts'