Add dropped items stats to LogStats

I wrote a few spiders, what crawling a very big sites with many items on many pages. I dropped already inserted items in pipelines. So, I want to have info about that. New log looks like this: 
INFO: Crawled 230 pages (at 60 pages/min), scraped 698 items (at 330 items/min), dropped 48698 items (at 30220 items/min)
INFO: Crawled 338 pages (at 31 pages/min), scraped 5141 items (at 1566 items/min), dropped 193327 items (at 26394 items/min)
This is the first my pull request, sorry if I made any mistake.
Part of code of process_item in Pipelines:

session.add(k)
        try:
            session.flush()
        except exc.IntegrityError:
            session.rollback()
            raise DropItem('duplicate "%s"' % item.get('some_part_or_item'))
        else:
            return item