TimeTable: fix departure board for calendar

In kraken all times are stored in UTC

however, for calendar we do not have a date to convert the time to a
local value (the conversion should be in jormungandr)

The for the calendar departure board api, we store in all vj the utc
offset needed to convert all stop times of the vehicle journey to local.

Thus:
* when queried with a date time the departure board answers in UTC and
the conversion is done in jormun
* when queried with a calendar the departure board answers in local
time
Merge pull request #6 from antoine-de/calendar_departure_board_tz

TimeTable: fix departure board for calendar