support redis cluster

[WingGao]

Let redis support cluster mode, both in broker and backend.
I haven't delete the original redis files, just add the new one by change the redisgo to go-redis

#config
broker: 'redis://host1:port1,host2:port2,host3:port3'
result_backend: 'redis://host1:port1,host2:port2,host3:port3'
...

Also fix this #145

[charleswhchan]

@WingGao : Can you please clarify the purpose of DelayedTasksKey for me?
What is the default if not specified in RedisConfig?

Thanks.

[WingGao]

The default value of DelayedTasksKey is delayed_tasks, it's here https://github.com/RichardKnop/machinery/blob/master/v1/brokers/redis/redis.go#L21 .
I made it to be configurable so that you can run multiple machinery instances for different projects in one redis db.

[charleswhchan]

Thanks @WingGao