Custom lookup support for jsonb @> containment op

I've implemented a Django lookup to expose the special jsonb `@>` containment operator. You can read more about jsonb containment in the 9.4 docs: http://www.postgresql.org/docs/9.4/static/datatype-json.html -- in short, you can query on subsets of the jsonb you're looking for (I've documented this in the code). With a GiN index on the jsonb field, these queries are lightning fast.

I'm probably going to implement more of the jsonb operators as I need them for a project I'm working on.