add: support for the read-write group

...h/systemsx/cisd/dbmigration/AbstractDatabaseAdminDAO.java

.../java/ch/systemsx/cisd/dbmigration/DBMigrationEngine.java

...stemsx/cisd/dbmigration/DatabaseConfigurationContext.java

.../java/ch/systemsx/cisd/dbmigration/IDatabaseAdminDAO.java

...urce/java/ch/systemsx/cisd/dbmigration/h2/H2AdminDAO.java

...temsx/cisd/dbmigration/postgresql/PostgreSQLAdminDAO.java

...msx/cisd/dbmigration/postgresql/PostgreSQLDAOFactory.java

...a/ch/systemsx/cisd/dbmigration/DBMigrationEngineTest.java