cubicweb-jsonb #16392301 Add Jsonb type [validation pending]

Make it possible to use Postgres jsonb type in CubicWeb. We should be able to write:

my_entity.cw_set(json_attribute={'a': 1, 'b': 2})

that is, to pass a dict as a value for a jsonb attribute. And conversely, using my_entity.json_attribute we should get back a dictionary.

done in0.1.0
load left0.000
closed by#9f494d936fd3 First README
patchFirst README [applied]Add psycopg2 dependency >= 2.5.4 [applied][schema] Create the Jsonb yams type [applied]