Allow setting referrer in a page download

This is what I did for #12.  It's not that elegant, but it worked.  I would say that expanding the API that's input to the download method makes more sense to me than overriding that method in subclasses of Scraper - what if I need a different referrer for each page?  The referrer is naturally a property of the page that's being downloaded, not the scraper.  So to handle more general settings, expanding the QueryBuilder seems like the right way to go.  Maybe creating a new user-facing class like PageSpec, or something, would be appropriate.