Added readme fetching & caching.

I have a feature enhancement I'd like to add to requirebin, but it requires not only browserified packages but access to npm readme files as well.

Rather than making a whole additional server, I patched my fork of browserify-cdn to feature a /readme/:module path to reply to GET requests with the html readmes right off of npm.

I offer it here to you to take or leave, I can always make mine its own server, but you can add it if you'd like:  thanks for such a great module!