JPA connector: Extension of JPA connector's configuration with default DB schema

Actually it is not possible to define a DB schema name on the JPA connector. Therefore I had to extend the JPASource with an additional instance variable and extend configure( Ejb3Configuration configuration ) method to set another hibernate property "hibernate.default_schema".

I think a defining of DB schema is a useful feature and it would be great to have it in JPASource in the future.