Add ability to use a unix socket for the REDIS_URL env var.

This also fixes Redis password handling and introduces a slight
backward incompatibility with the format to ease parsing.

Old:

    redis://password@host:port/db

New:

    redis://:password@host:port/db
Merge remote-tracking branch 'jezdez/redis-unixsockets'

Conflicts:
	feedhq/settings.py