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.
Mmh, I thought I added a comment here.
Use [`findSplitBefore`](http://dlang.org/phobos/std_algorithm.html#findSplitBefore) to avoid the temporary array created by split.