Add GTFS import test, fix calendar table

- Use Google's sample GTFS feed for the test
- Fix 'weekdays' field in calendar table, which combines GTFS fields
'monday'-'sunday' into a string of 1's and 0's
- Fix conversion of dates to be stored in the database. It now stores
dates as strings, storing QDates directly does not work
- Fix target directory for extracted GTFS feeds, ie. do not use the
path of the zip file for the directory name, which may include double
dots and thus change the path away from /tmp
- Fix filling missing fields with invalid QVariants in
GeneratTransitFeedImporter::readFields() (if => while)