Improve error handling, progress reporting, non-blocking import

- Report progress/errors of importing/downloading GTFS feeds through
the data engine to visualizations
- Add optional <timeZone>-tag to accessor XMLs, can be used to define
a timezone to use, if none is given otherwise. Used by GTFS accessors
if the imported feed doesn't include timezone information in
agency.txt (although required for valid GTFS).
- Create new class GeneralTransitFeedDatabase, which manages GTFS
databases
- Do importing of GTFS feeds in a separate thread to not block the
whole data engine and thus whole plasma
- Check required files in GTFS feeds before importing them
- Check minimum records for GTFS feed files (eg. at least one stop)