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.
Yeah, we can definitely do that. Then we could make this function specific to downloads.

Though I'm not sure how that works with the router? I'm assuming the code would look something like this, but I can't get it to compile.
```d
router.get("/api", new ApiWebFrontend);
```