Model: add missing companion objects
Model: make `Model` a class not a trait
Model: improve existing companion objects and factories
application.conf: add H2 database settings

build.sbt: add hibernate as dependency
Tests: adopt tests of service classes to latest changes
Model: add JPA annotations to model classes
Database: add persistence.xml
Database: add *.db files to .gitignore
Conference: fix bug in mappedBy parameter
Database: fix wong hibernate version in dependencies