Use Mapnik to read GeoJSON rather than OGR

[springmeyer]

We should start using new mapnik.Datasource({'type':'geojson'.... instead of 'type':'ogr'.

Using 'type':'geojson' triggers using https://github.com/mapnik/mapnik/tree/master/plugins/input/geojson instead of https://github.com/mapnik/mapnik/tree/master/plugins/input/ogr. We use the Mapnik OGR plugin currently in Omnivore because when @GretaCB started mapnik-omnivore the native Mapnik GeoJSON plugin still had a few bugs. But those bugs are now long ago solved and this GeoJSON plugin is in heavy use for the Mapbox Static Maps Overlay API (https://www.mapbox.com/developers/api/static/#overlay) without any reports of problems.

The advantage of the Mapnik native GeoJSON plugin is that is it faster and uses less memory.

With a 100 MB GeoJSON of US county boundaries I've seen OGR uses > 2 GB and takes > 6 seconds to open the file (even before tile rendering). Mapnik's native GeoJSON plugin requires just 1-200 MB and reads the same file in .8 seconds. (@artemp also has ideas to speed up GeoJSON reading even more: mapnik/mapnik#2636).

[springmeyer]

done in #67 by @rclark