Better support of times & timezones

- use `LocalDateTime` instead of `DateTime' for client facing timestamps.  these values are just a timestamp without any timezone (conference fields: registration start & end times, event start & end times, early registration cutoff
- upgrade to latest sql2o version which adds support for `LocalDateTime`
- use separate Json serializers for `DateTime` and `LocalDateTime`
I would use `org.joda.time.DateTimeZone` for `eventTimeZone` here and in `ConferenceEntity`. Might need to write serializer/deserializer for Jackson. sql2o layer might need customization support as well.