Link in description & option to invert link/permalink.

Patch for issue https://github.com/sebsauvage/Shaarli/issues/19

Now:
* The (perma)link is added at the bottom of description.
* If "permalinks" is added in URL parameters, link/permalinks will be
swapped.

eg.
* Normal link in title + permalink in description:
http://mysite.com/shaarli/?do=rss
* Permalink in title + normal link in description :
http://mysite.com/shaarli/?do=rss&permalinks

It works for the ATOM feed too.

(Happy ?   :-D  )