Fix relations for RDBMS connectors (mysql, postgresql)

- fix tests (explicit model/property definitions)
- fix include vs. RDBMS model strictness
Merge pull request #237 from fabien/fix/rdbms-relations

Fix relations for RDBMS connectors (mysql, postgresql)
Implement DAO unsetAttribute
Coerce embedded model types
Implemented embedsOne
Validate embedded models by default
Fix test cases
Merge branch 'master' into fabien-feature/embeds-one