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 am no fan of `@tparam` and `@param` (not saying they are not useful at time). They don't bring much and just prove that the description was not good enough.

Would the following be ok to you?

```
/**
* A typeclass for graph stores supporting [RDF Datasets](http://www.w3.org/TR/rdf11-concepts/#h2_section-dataset) manipulations.
*
* The store of type `A` can manipulate named graphs (no default graph) through their names (only URIs, no bnode). Operations happen inside the context `M`.
*/
```