Gtfswrite

Fix #1630 (GTFS write functionality).
How about this as a replacement:

```java
    Iterator<CalendarDate> iterator() {
        Iterator<Service> serviceIterator = feed.services.values().iterator();
        return Iterators.concat(Iterators.transform(serviceIterator, new Function<Service, Iterator<CalendarDate>> () {
            @Override
            public Iterator<CalendarDate> apply(Service service) {
                return service.calendar_dates.values().iterator();
            }
        }));
    }

```