Add of GUID field in History RSS feed

The NZO id is used as unique id for the history RSS feed to help some RSS readers (like Thunderbird) to identify articles when the link field is the same for all articles.
Add of GUID field in Queue RSS feed

The NZO id is used as unique id for the queue RSS feed to help some RSS
readers (like Thunderbird) to identify articles when the link field is
the same for all articles
Merge pull request #104 from manandre/rss_guid

Add of GUID field in History and Queue RSS feeds