Merge: neo4j: Proof of concept for JSON serialization.

This a quick and dirty proof of concept for JSON serialization of graphs (see issue #853). After the refactoring of the JSON APIs and of the graph APIs (@Morriar and @xymus know what I’m talking about), there will be a cleaner way to do this. But, because the promised APIs take more time than expected to design, I implemented a working solution with what it is already there to give you at least a rough idea of the serialization logic that I have in mind.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>

Pull-Request: #890
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>