Adapt to latest vector tile spec

 - pulls in the vector tile v1.0.1 spec (+ adds back experimental raster)
 - moves to separate the `vector_tile` spec/protobuf namespace from the `mapnik::vector_tile_impl` on top of it.

Helper to cope with this renaming:

```sh
for i in {"src","test","examples"}; do
    find $i -type f -name '*.cpp' -or -name '*.h' -or -name '*.hpp' | xargs perl -i -p -e "s/mapnik::vector::tile_layer/vector_tile::Tile_Layer/g"
    find $i -type f -name '*.cpp' -or -name '*.h' -or -name '*.hpp' | xargs perl -i -p -e "s/mapnik::vector::tile_feature/vector_tile::Tile_Feature/g"
    find $i -type f -name '*.cpp' -or -name '*.h' -or -name '*.hpp' | xargs perl -i -p -e "s/tile_layer/vector_tile::Tile_Layer/g"
    find $i -type f -name '*.cpp' -or -name '*.h' -or -name '*.hpp' | xargs perl -i -p -e "s/tile_feature/vector_tile::Tile_Feature/g"
    find $i -type f -name '*.cpp' -or -name '*.h' -or -name '*.hpp' | xargs perl -i -p -e "s/mapnik::vector::tile_value/vector_tile::Tile_Value/g"
    find $i -type f -name '*.cpp' -or -name '*.h' -or -name '*.hpp' | xargs perl -i -p -e "s/tile_value/vector_tile::Tile_Value/g"
    find $i -type f -name '*.cpp' -or -name '*.h' -or -name '*.hpp' | xargs perl -i -p -e "s/mapnik::vector::tile/vector_tile::Tile/g"
    find $i -type f -name '*.cpp' -or -name '*.h' -or -name '*.hpp' | xargs perl -i -p -e "s/tile_GeomType/vector_tile::Tile_GeomType/g"
done
```