2 Fixes: deleteReadOlderThanThreshold calculation fix and multiple XPath results for articleenhancer

<b>deleteReadOlderThanThreshold calculation fix:</b>
I noticed that there is a defect in the calculation of the limit in deleteReadOlderThanThreshold(), leading in no deletion of "old" items at all:
* `$limit = $row['size'] - $threshold;` will always lead in $limit < 0 because the SQL-statement selects only feeds with more than $threshold unread-items `(HAVING COUNT(*) > $threshold)`.
* But the deletion only happens `if($limit > 0)`.

So in my opinion, the calculation of $limit has to be just reverse. Can you confirm that?

<b>multiple XPath results for articleenhancer</b>
ArticleEnhancer is a killer feature! I like it very much! Thanks for implementing that! This is a litte improvement that allows to extract all the results of a XPath-term instead of only the first.
So we can use for example:
`'/explosm.net\/comics/' =>
 "//*[@id='maincontent']/div[2]/div/img | //*[@id='maincontent']/div[2]//td[1]/nobr"`