Allow the Entityhub to use native Sesame RDF serializers

With the current available serializes instances of the Sesame based Entityhub Model implementation would be (1) converted to the Clerezza based Model instance and (2) get serialized by using the Jena serializes via the Clerezza/Jena model adapter.

This will

• improve/extend the Entityhub to select Serializer based on the native type of the used model
• provide Serializer for Sesame model implementation

It needs also to be checked if this extension can be back ported to 0.12.0 as this branch uses a considerable different architecture.