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.
I was trying to keep the DB layer as simple as possible, but I think the date filtering makes sense. That said, I'm not quite sure how to structure that query. When I use the following:
```d
[
	"package_": pack,
	"time": ["$gte": since]
]
```
I get the following error:
```
Error: incompatible types for ((pack) : (["$gte":since])): 'BsonObjectID' and 'SysTime[string]'
```
Is there some intermediate type I can reduce them to, like JSON? I don't know the MongoDB API very well.