Replace download_natural_earth_data with make

This caches Natural Earth zips in data/ to prevent unnecessary,
time-consuming downloads.

This also makes it possible to bring up a new region via

    make db/belize

including all dependencies.
Clean up data Dockerfile
Split db targets to facilitate data volume construction
TODO
Improved error handling / isolation
Merge pull request #23 from stamen/refactor-build

Refactor build