Bug fix and performance improvements:

1. Fixed bug in city migration where if there were two cities in the same country with the same name only the first would be imported. This was due to not taking into account the region checking for the records existence before creating a new one.
2. Increase performance by caching region_id and country_id in migration function variables.
3. Added debugging flag '--verify-city-import' which cerifies city import and print out missing cities. For debug purposes only.
4. Fixed bug in city migration where if the name_ascii field is null it would set the slug name as 'city.' It now sets it to the geoname_id.