Use Onionoo instead of hard-coded GeoIP database.

Instead of shipping an outdated GeoIP database and using crappy code to
search the database, we should query Onionoo to learn about all relays
in a specific country.

This should fix <https://github.com/NullHypothesis/exitmap/issues/6>.
Remove old GeoIP code.

The GeoIP database is huge and we are now free to remove it.  Also, move
the geo location code to the util module.