rss: Added rss destination module.

A very simple destination module, which can offer log messages as
an rss feed.

Example config:

destination d_rss {
   rss(
     port(5555)
     feed_title("RSS")
     entry_title("$MESSAGE")
     entry_description("$MESSAGE")
   );
};

Signed-off-by: Tusa Viktor <tusavik@gmail.com>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>