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`
:+1: At first I thought these `JsonIgnore` annotated methods might be better declared as private within `RegistrationWindowCalculator` so you wouldn't have to bother about the `JsonIgnore`. However, as they do seem more appropriate on the `Conference` object, I prefer this as well.