improve tests:
 - be explicit about mercator projection
 - start testing images visually
 - fix visual image by attaching style properly
improve tests
repair Mapnik 2.3.x compile
switch to gdal plugin since PPA does not support raster.input
Merge branch 'master' of github.com:mapbox/mapnik-vector-tile into raster-clip
fix test rebuilds
Merge branch 'master' of github.com:mapbox/mapnik-vector-tile into raster-clip
adapt to latest Mapnik 3.x
Merge branch 'master' of github.com:mapbox/mapnik-vector-tile into raster-clip
adapt to mapnik 3.x