Compatibility with WebAnnotation (JSON-LD)

[christian-oreilly]

The format of the annotations should be adapted to make them compatible with the WebAnnotation format (https://www.w3.org/TR/annotation-model/). This standard support JSON-LD annotations, which could be used to extend the current NAT JSON data structure. This would make it easier to be inter-operable with other tools such as hypothes.is and with text-mining project such as OpenMinTeD.