Add 'scale factor' option to Mapnik image provider

Adds the optional 'scale factor' argument in the config for Mapnik's
image provider. Allows a TileStache layer to be configured for retina.

This allows specifying the 'scale_factor' option the ImageProvider. It's used to render retina tiles.

For discussions about it, see:
https://github.com/mapnik/mapnik/wiki/Scale-factor
http://support.mapbox.com/discussions/mapbox-ios-sdk/275-retina-maps-for-completely-custom-style
https://groups.google.com/forum/#!topic/mapnik/g4MiIZu9SCk