[jena] Adding JSON-LD Writer instances

Implement JSON-LD writing for Jena :
    a no-brainer

￼Added some documentation for Generic Parameters, with @tparam :
    it is actually pasted in 3 files; maybe do that only in RDFStore.scala ?
Is there a reason why you choose `JsonLdCompacted`? Is that really what's happening? If yes, can you add a comment to the relevant documentation in Jena? If you are not choose, I would really prefer to see `JsonLd` anyway.