Fix invalid layers problem

mapnik.Map objects were being re-used and accumulating more layers than
they should have (with non-matching extents).
GeoJSON → PBF data tiles

Added TileJSON for [Mapnik Vector
Tiles](http://openstreetmap.us/~migurski/vector-datasource/).

Also includes various other tweaks to limit the number of upstream tiles
requested.
Adapted TM2 style