gtfs - volanbusz stops

[bhollosi]

• data_handlers: city, street, poi, patch, insert only
• data_structure: new type: bus_stop
• data_structure: human-readable str representation for poi patch & country
• hu_volanbusz: wip: stops
• poi_datase: caching result of street name setter
• poi_patch: removed duplicated lines
• soup: check etag http header of response, skips re-download if the locally stored value matches the remote header value