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`
Shouldn't you fix this now? I suppose it's a matter of converting 'conferenceCostsEntity.getEarlyRegistrationCutoff()'  to `DateTimeZone` by way of [LocalDateTime.toDateTime(DateTimeZone)](http://joda-time.sourceforge.net/apidocs/org/joda/time/LocalDateTime.html#toDateTime(org.joda.time.DateTimeZone)).