raven/models/Feed: disable pubsubhubbub for now

We don't have it working yet, and it's causing distracting splats.
raven: proper calculation of last_fetched

We previously had a hack of touching every feed when it got added to
the fetch queue, to control how often feeds got entered into the queue.
This scheme didn't work, and we often ended up fetching the same feed
multiple times within a few seconds when attempting to clear out the
queue.

Instead:
	- update Feed.last_fetched only after a successful fetch.
	- have Feed.update() compare last_fetched vs fetch_frequency and
	  return if it's too soon
raven/models/Feed: do less work in update()

Add some more conditions where we shouldn't waste our time fetching
feeds in update().
raven/models/Feed: start handling bozos

Use the bozo bit to start marking feeds as dead after we attempt to fetch.

For now, only handle errno -2, as these are truly dead feeds.
	<urlopen error [Errno -2] Name or service not known>
raven: more informative logging everywhere

Better logging.
raven/tasks: twiddle fetch updates and sizes

In our continued quest for optimizing fetch frequency vs. size, trying to
go for fewer celery tasks, but each one doing more.
Merge pull request #13 from achiang/master

battle scars