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.
Properly document 'scale factor' option.

Remove improper underscore
Merge remote-tracking branch 'refs/remotes/jwass/scale_factor'

* refs/remotes/jwass/scale_factor:
  Properly document 'scale factor' option.
  Add 'scale factor' option to Mapnik image provider