Group submissions of requests to operations queue.

This is a follow-up on issue #348 : grouping the requests submissions greatly reduces the number of notifications being posted when the operation queue switches between empty and non empty state.
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.
Fix issue #348

Make sure that we are really handling a refresh of feeds (and not another kind of network activity) before posting a MA_Notify_RefreshStatus notification
Merge pull request #357 from barijaona/issue-348

Fix issue #348
Merge pull request #355 from barijaona/bugfixes

Fix repeating articles issue with the Netflix feed