Integrate N3.js (node-n3) parser directly with rdfstore-js

Currently, [N3.js (node-n3)](https://github.com/RubenVerborgh/N3.js) has been added to rdfstore-js in an ad-hoc way.
Unfortunately, this means that it is **difficult to update** the version; witnessed by the fact that it is [seriously outdated](https://github.com/antoniogarrote/rdfstore-js/pull/49) at this moment. rdfstore-js still uses the instable 0.1.1 version, whereas the stable 0.2.0 has just been released.

In other words, rdfstore-js comes with **several parsing bugs** that have already been fixed.

I would volunteer to **add N3.js as a submodule** to the project and integrate it with the rest of the code, making obtaining the latest version as easy as `git submodule update`.

However, I'd like @antoniogarrote's approval before I start working on this.
And comments are also welcome, for instance, on the adaptations that were performed for the current integration.