Use AgencyTable instead of a list of AgencyRow in GtfsObject.
Add some code to CalendarDatesTable do read calendar_dates.txt.
Refactor: crreate a GtfsTable class which each *Table should extend.
Refactor: moves all the *Tables to gtfs_table.dart
Fix GtfsObject.hashCode: the result was not returned.
Rename GtfsObject GfsRow. It is more consistent with GtfsTable.
Refactor: moves all the *Row to gtfs_object.dart.
Rename gtfs_table to gtfs_tables and gtfs_object to gtfs_rows to be more consistent.
Refactor: use CalendarDatesTable to read CalendarRows in Gtfs.