support for HiDPI

The module doesn't seem to works with HiDPI screens like Retina.

Example:
save following under config.json, then run /usr/bin/nodejs /usr/bin/tessera -c . -p 28089

{
 "/toner": {
   "source": "tilejson+http://staging.tile.stamen.com/toner-background/index.json"
  },
 "/toner-http": {
   "source": "http://toner-elb-3vaew2i23b6d-1666871272.us-east-1.elb.amazonaws.com/toner-background/{z}/{x}/{y}.png"
  }
}

http://127.0.0.1:28089/toner-http/6/11/26@2x.png returns a 512x512 image. Fine.

http://toner-elb-3vaew2i23b6d-1666871272.us-east-1.elb.amazonaws.com/toner-background/6/11/26@2x.png returns a 512x512 image (it's the link described in the tilejson file). Fine.

http://127.0.0.1:28089/toner/6/11/26@2x.png returns a 256x256 image. Wrong :-(.

The solution is probably as simple as integrating the modifications proposed in https://github.com/mapbox/node-tilejson/pull/26/files.

Beginne damit, das Verhalten mit der bereitgestellten config.json und den /toner- und /toner-http-@2x-URLs zu reproduzieren. Vergleiche das Modul mit den im verknüpften node-tilejson Pull Request vorgeschlagenen Änderungen. Fertig ist die Aufgabe, wenn der /toner/6/11/26@2x.png-Endpunkt das erwartete 512x512-Bild statt eines 256x256-Bildes zurückgibt.