Time zone


# DO NOT MERGE THIS PR, IT'S JUST FOR REVIEW FOR THE MOMENT

## First PR for time zone handling

The main idea is to convert ALL times to UTC  in the import process, handle only UTC in kraken (the computation part) and change it to local time only at the end of the process in Jörmungandr (the middleware) using the added time zone info in each stop areas.


This PR handles the GTFS parser part.

The UTC conversion difficult part is day saving time (DST) handling.  
For example a stoptime at 7am valid over the year in a paris (Europe/Paris timezone) will be 6AM UTC in winter but 5AM UTC in summer.

Thus all validity_pattern (gtfs calendar), vehicule_journey (gtfs trips) and stoptime have to be split over the DST period.

## Note
despite all its flaws we use boost::date_time for timzeone handling.
for the moment the boost csv is hardlinked in the project, but it will change.




for null values I think it should be handled in the conversion.

For the field existence, we never check it (just above but I think it's historical and useless since we don't check the shape).

You could decide to always (or with all polling request) check the existence for compatibility reason, but that won't be the only thing to change