Make HibernateInterceptor replaceable

[flybyray]

The (static) HibernateInterceptor needs to be configurable. Either through play plugin system or through injecting an own implementation of the hibernate interceptor. the latter is preferred as it will not introduce new PlayPlugin methods.

Background: If someone needs to integrate deeply with a process engine transaction handling can be tricky, and some additional events from hibernate need to be accessible.

Just some simple modifications arround that line would be great: https://github.com/playframework/play1/blob/master/framework/src/play/db/jpa/JPAPlugin.java#L179