Add marshalling hooks in Jaxb2RootElementHttpMessageConverter

Allow Jaxb2RootElementHttpMessageConverter subclasses to customize
the {@link Marshaller} and the {@link Unmarshaller} created by the
message converter.

Issue: SPR-11488
Move customize(Un)Marshaller methods to abstract class

Issue: SPR-11488
Merge pull request #474 from sdeleuze/SPR-11488