river.py: track state in redis and drop provided timestamps

Track what we've seen before via the 'fingerprints' set and keep what
will eventually become the river in the 'entries' list.
entry_fingerprint: don't hash pubDate/description

title/link/guid are much more stable, from my experience
river.py: add an extra linebreak before announcing elapsed time
entry_fingerprint: encode as a separate step
river.py: add --initial flag

When ran with this flag, only entries published within the past 24
hours are included in the river.

This gives more feeds a chance to get onto the river and prevents
feeds with a lot of entries from drowing all others out.

Eventually I'd like this 'only show recent' behavior to kick in
whenever a new feed URL is discovered and to have the number of hours
configurable.