Mark/neoid with json field support

I was working on an application that runs dual db: PostgreSQL 9.2 and Neo4j, where a model have a __json-type field__. This wasn't able translatable using the original neoidable config.

So I wrote added a __json_field__ definition that allows json-type fields to be used in Neoid. Basically this will add the content of the json_field to neo4j node.

A drawback is that I can't test this on sqlite3 in-memory db, so I have to create a local postgres db to test. I am open to suggestions on this, though