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.
Not quite sure what you mean by this. My revised code looks like this:
```d
cast(uint)m_db.getPackageDownloads(pack._id, currTime - numDays.days).filter!(dl => dl.version_[0] != '~').walkLength()
```
Is that what you meant?