Add downloads visualization for index and package

This adds the two simple visualizations for index and package download counts. Sorry for the use of semicolons in advance.

I tried to get this to run directly off of http://isaacs.iriscouch.com/downloads using CouchDB's JSONP support but the problem is that on https, the certificate that gets served from there is a self-signed cert for registry.npmjs.org, so even with JSONP the request don't go through directly (since the browser will block them if either the protocol does not match e.g. https -> http or because the cert authority is unknown). So I wrote proxy endpoints instead. 

Let me know what you think @isaacs