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 the compiler telling you have incompatible value types in your hash.
Basically you need to convert everything to `Bson[string]`. http://vibed.org/api/vibe.data.bson/Bson.this

This should do the trick.
```d
return m_downloads.find!DbPackageDownload(
	[
		"package_": Bson(pack),
		"time": Bson(["$gte": Bson(BsonDate(since))])
	]);
```