Added package stats route with download count

The idea is that this should allow us to start adding badges to [shields.io](http://shields.io/).

This PR also leads the way towards adding more stats in the future.
That's for plain handlers. Apparently whole controllers are registered using 	`router.registerWebInterface(webfrontend);` like [here](https://github.com/D-Programming-Language/dub-registry/blob/aa9e9ab37fb4e6c18a8d91720e8bb401e53b8255/source/dubregistry/web.d#L28).
So there should be an api.d file that contains a `void registerDubRegistryAPI(URLRouter router, DubRegistry registry)` which gets called in `source/app.d`.