[model] util: Add new DateTimeConverter helper

Convert from org.joda.time.DateTime attribute to
java.sql.Timestamp
[jpa] add orm.xml with converter specs for DateTimeConverter
[model] Conference: +{start, end, deadline, logo, thumbnail }

Add new fields to conference
[service] Conference: order list/listOwn by startDate
Merge pull request #76 from gicmo/ConferenceModelChanges

Conference: +{start, end, deadline, logo, thumbnail } fields