Radical

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 ?
I haven't looked in the Jena API but I don't understand how the 3 versions are actually different writers. You always pass the same `RDFLanguages.JSONLD`.