get_url: fix cache in the case of redirect

this still is sort of smelly and probably needs a rethink...
write_if_change: don't touch output files if there's no difference
index.html: produce an index of generated feeds
don't crash on non-ascii titles in debug log
Assume unicode URLs can be UTF-8 encoded
Fix href attribute of link tag
When ID is not available, use Link
Fix authors in feed
Fix authors in entry
Fix link in entry
Use link as entry id when id not available