tool/downloader.rb: split particular sites

* tool/downloader.rb (Downloader): split particular sites from the
  main class.

* tool/downloader.rb (Downloader.download): show messages if
  verbose mode.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47725 b2dd03c8-39d4-4d8f-98ff-823fe69b080e