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`
It was a pain.  The tests would run fine in my environment, but fail on jenkins for an unknown reason.  The value added from the tests did not come near the headache created to figure out what JVM settings were different or apply a hack to have the tests work.