Configure Redis cache for all environments

Also add support for connecting to Redis with Unix sockets.
Store sessions in a Redis namespace

This makes less of a mess of the Redis root.
Rename resque_url to redis_url_string
Hide configuration shenanigans in a block

I'm feeling paranoid about the scope variables like redis_config_file
get defined in. Hiding it in a block to limit the scope.
Add CHANGELOG entries for Redis fixes
Merge branch 'redis_settings'

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Conflicts:
	CHANGELOG