entry_fingerprint: don't include feed url

No matter what feed it comes from, if two entries have the same title,
link, and guid they should only be displayed once in the river. It
doesn't matter their source feed URL.

This really helps the New York Times river as they include the same
story in multiple feeds.
download.py: don't check for feed cache existence

Just grab and if it wasn't there go ahead and populate the cache.
download.py: cache for 15 minutes

Might lower this later, but for now 15 will work.
river.py: add --clear argument

In combination with --source, this deletes everything (*) we've previously
seen about this feed.

This treats OPML feeds as if we've never seen them before.

(*) With one exception: 'riverpy:feed_cache' isn't touched.
river.py: add --clear-all argument

Like --clear, but operates on sources.
top100.opml: remove Ars and 37 Signals feeds