jsonalchemy: JSON-LD addition

• Add utilities for serializing SmartJson objects to JSON-LD. See specification

[1] and the used pyld library [2]. The recommended method for publishing
JSON-LD is via the REST extension.

[1] http://www.w3.org/TR/json-ld/
[2] https://github.com/digitalbazaar/pyld

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>