Include GeoNames to verify admin boundaries

[mattigrthr]

We are currently building the admin boundary hierarchy of a country based on OSM data. One common use case is to group data by city. Since the number of the admin levels on OSM have different meanings in different countries (e.g., admin level 8 might be a city in the US, but in Germany admin level 6 might be used for cities), we should include another data source for determining city names.

GeoNames is an open database of worldwide country and city names. We can use their data dumps to get relevant city names of countries worldwide and match them with our OSM admin boundaries.