Add comment method to QuerySet.

This allows SQL comments to be added to django queries.

Ex:
comment = 'Hello DB'
Book.objects.filter(id=1).comment(comment)

SQL output:
SELECT "book"."id" FROM "book" WHERE "book"."id" = 1 /* Hello DB */

We aren't necessarily expecting to have this merged into 1.4 (we already have it locally) but we are interested in merging this into master and were curious to see if this is something the django team is interested in.