Provide Import/export to RDF to ensure compatibility with LinkedData

[jibe-b]

To ensure compatibility with semantic graphs out of the box, there is a need to serialize exports and parse imports to/from RDF syntaxes.

Here are the features that would be needed by
https://github.com/jmvanel/semantic_forms to have Semantic Forms and Rhizi compatible:

• export to RDF (Turtle and / or JSON-LD ) , using classic vocabularies like FOAF and schema.org
  REST API, and / or
  dump of the data
• import of RDF data in Rhizi
• drop of RDF document URL
• drop of RDF string

[jibe-b]

cc @jmvanel