Refs #268. Improves performance of fetch_urls by using django-queryset-iterator to process large TwitterUserItem QuerySets in batches, using HEAD falling back to streaming GET requests to avoid pulling down the response body, and optimizing refetching. Also, restricted the version of requests to avoid an infinite redirect loop bug, added catching more specific exceptions and printing the URLs and error messages, setting a 10 second timeout on HTTP requests, and starting with the Twitter API-provided expanded_url, where possible.
Merge branch 'cazzerson-master'