ip: Add 'reallyfreegeoip' method
This method uses the JSON API of reallyfreegeoip.org to determine a city-level location.
The API does not provide an accuracy estimate, so accuracy is hard-coded to a value (20 km) below the city level location threshold (15 km). This seems to be on the safe side (not too small) from testing with N=1.
Depends on !186 (merged).