added toString() methods to feed classes which makes it possible to
export full rss feed files out of the RSSFeed class
added rss feed output to snapshot servlet which can be used to get a
list of latest/oldest entries in the snapshot database. This is an
example:
http://localhost:8090/api/snapshot.rss?depth=2&order=LATESTFIRST&host=yacy.net&maxcount=100

The properties depth, order, host and maxcount can be omited. The
meaning of the fields are:
host: select only urls from this host or all, if not given
depth: select only urls at that crawl depth or all, if not given
maxcount: select at most the given number of urls or 10, if not given
order: either LATESTFIRST to select the youngest entries, OLDESTFIRST to
select the first entries or ANY to select any

The rss feed needs administration rights to work, a call to this servlet
with rss extension must attach login credentials.