Allow attribs to work with the Namespace gem...

Changed the setter and getter to work like Redis::Objects because I'd like Attribs to use the Namespaced connection in a similar way to Redis::Objects. In my redis.rb I'm settting them like this:

$redis = Redis::Namespace.new("blah", redis: Redis.new(:host => uri.host, :port => uri.port, :password => uri.password))

Redis::Objects.redis = $redis
Redis::Attrs.redis   = $redis