settings: enable multiple databases, which allows for postgres streaming

A combo of these pages:
	http://nxvl.blogspot.com/2012/10/multiple-databases-in-django.html
	http://blog.thecodepath.com/2012/02/13/adventures-in-scaling-part-3-postgresql-streaming-replication/

One gotcha, in Ubuntu 12.04, if using the postgres 9.2 PPA, be sure to
apt-get install postgresql-contrib, else you will not have the
pg_archivecleanup command, and you will fill up your disks!
raven/models/Feed: tune fetching frequencies

First, only fetch every 10m instead of every 5m, as that was overwhelming
our ability to keep up.

Next, let's be more selective about 'fast' feeds. Only allow feeds with
multiple subscribers to be fast. Also, add some smarts about detecting
feeds that started off fast, but have since slowed down.

Finally, look for dead feeds. Only warn about them since we want to be
fairly conservative about whether to really stop fetching or not.
Merge pull request #2 from achiang/the_grind

backend tuning