Drop into a while loop to keep checking feeds

Previously, running river would check all the feeds, write the
river.js files, and quit.

Now, river idles in a while loop waiting for feeds to come up again
for a check.
Use a moving average to set the update interval

Look at a feed's update history and use that to set its next update.
Display the feeds that are next up for a check
Swap count/feed_url when logging new entries
Add river-writer command

The "sub" of the pubsub system that listens for messages on the
'update' channel and upon receipt generates and uploads river.js files.
Don't double the size of the firehose feed
Merge branch 'pubsub'
Drop serialize_manifest function
Ignore generated rivers
Remove serialize_manifest import
Don't keep retrying failed feeds

If requesting a feed fails, check again in an hour.
Generate manifest.js file
Merge branch 'build-manifest'
Drop logging to a file

This was more necessary when ran via crontab as I had no other way of
checking how things were going. But with the new system that drops
into a main loop, makes less sense.
Use formatted timestamp for next check when feed fails
Cap next checks at 1 minute to 2 hours
Use local time for timestamp in failed feeds
Drop angular-moment for timestamps
Add refresh icon