Merge pull request #354 from barijaona/bugfixes

Follow-up to issue #348 : group submissions of requests to operations queue.
Modified workaround for repeating guids

As discussed at <<http://forums.cocoaforge.com/viewtopic.php?t=26612> : the Netflix feed does not include dates, and has the last movie repeated in the feed. So a guid value is duplicated within the feed, which is invalid.
Other problematic feed, like this one <http://depeches.orange.mg/fluxrss.xml>, have duplicate guids and ambiguous titles and links ; but the date can be usefully used to discriminate between different articles.