Add support to query builder for creating sort arguments

[anthonyjb]

At the moment to specify the order of a set of results we'd do something like:

articles = Ariticle.many(sort=[('publish_date', DESC), ('title', ASC)])

I think it would be a useful addition to the query builder if we would instead build these sort lists along the lines that we build queries, e.g:

articles = Ariticle.many(sort=[Q.publish_date.desc, Q.title])

[anthonyjb]

This functionality is now supported using the SortBy function, for example:

articles = Ariticle.many(sort=SortBy(Q.publish_date.desc, Q.title))