title: Enabling deltafetch in scrapy
body: <p>I have worked on <code>scrapy</code> a bit and now I have my spider ready. But now I want my spider to scrape only those items which is not been scraped in its previous run, and scrape only the new contents. By achieving this I can reduce the runtime of my spider. </p>
  
  <p>While studying about this I came across <a href="https://github.com/scrapinghub/scrapylib/blob/master/scrapylib/deltafetch.py" rel="nofollow">deltafetch</a>, Which I think will serve my requirement. But I am not being able to import that feature. I would be glad if any body could guide me about using it in a well defined way. </p>
  
  <p>And also if there is any other middleware which serve the similar purpose I would be interested to know.   </p>