Support natively data classes + repository based DAO

As demonstrated in the [geospatial-messenger](https://github.com/sdeleuze/geospatial-messenger) application, data classes + repository based DAO are really nice to use, I think Exposed could help to use this pattern with proper documentation and maybe provide some helpers (mapper/unmapper).

See [Database.kt](https://github.com/sdeleuze/geospatial-messenger/blob/master/src/main/kotlin/io/spring/messenger/Database.kt) and [DefaultMessageRepository](https://github.com/sdeleuze/geospatial-messenger/blob/master/src/main/kotlin/io/spring/messenger/repository/DefaultMessageRepository.kt) for more details.