Include `RedisCluster` class

[tPl0ch]

Currently the RedisCluster class is not exported via extension. Maybe we can add this at least into the stubs here in the master branch at least.

[ukko]

Hi, sorry, I stopped using Redis and don't know what's going on with the.

If you can send me the patch with the corrections I gladly accept it.
Also, you can fork this project and modify on their own.