Adds Resque::Config#initialize

* Explicitly pass a Redis-like to config.redis in Resque#redis= instead of doing so implicitly, making it easier to test
* Move Redis::Backend::connect tests from config_test.rb to backend_test.rb
* Move/add Resque#redis= docs to Redis::Backend::connect

Props to @yaauie for pairing with me and writing most of this
Rearrange config attr_accessor :redis
Remove trailing whitespace
Merge pull request #1055 from edward/refactor-config-and-resque-singleton

Adds Resque::Config#initialize