Add of GUID field in History and Queue RSS feeds

This is the fix to an issue that I reported in SABnzbs forums:
<p><FONT SIZE=24><a href="http://forums.sabnzbd.org/viewtopic.php?f=3&t=14439">Thunderbird only displays the first item in History RSS feed</a></FONT></p>
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The NZO id is used as unique id for the history and queue RSS feeds to help some RSS readers (like Thunderbird) to identify articles when the link field is the same for all articles.