A new utility method, prefix_q, has been added to facilitate

prefixing of query expressions. Query expressions, or (Q()

expressions) are Django's way of building the WHERE (and related)

clauses when querying for models.
If a foreign key fk exists from a model A to B (that is, A.fk

points at an instance of B), then a query expression that is used

to filter instances of B can be used with A via prefix_q:
q_a = prefix_q('fk', q_b)

This allows complex queries that select one model to be reused to

select related models very easily.

Ran unit tests.