Added jsonb support

Added jsonb support, at the same level of functionality as the current json support. All json tests are running successfully on jsonb with Django 1.7rc1 and PostgreSQL 9.4 beta.

Next steps: Look into adding transforms / lookups so that we can use more of the special GiN-accelerated jsonb operators, such as containment `@>`, existence `?` and so on. See http://www.postgresql.org/docs/9.4/static/datatype-json.html