adopt RDFJS interfaces

[elf-pavlik]

W3C RDFJS CG created through community process a proposal for common RDF interfaces for JavaScript. You can find it in https://github.com/rdfjs/representation-task-force/blob/master/interface-spec.md

Many libraries started adopting this interface and providing feedback before the TF will finalize the spec: rdfjs/data-model-spec#67

[jacoscaz]

Hello @antoniogarrote . Over the last few weeks we've been working on https://github.com/beautifulinteractions/node-quadstore, a quadstore for both Node.js and browsers that supports the RDF/JS Store interface. Its indexing strategy looks almost identical to rdfstore-js's.

I've had a look at your code and it looks to me like extending QuadBackend (#134) to support the RDF/JS Store interface would allow rdfstore-js to implement persistent storage in any context where a suitable implementation of such interface is available, Node.js included (#6).

Would you accept a PR for this?