Implements undocumented jedis cluster commands

PR is based on https://groups.google.com/d/msg/jedis_redis/5h8mMyq2CUI/ee6LgYcvvTsJ.
(And #579, too)

I've posted about discussion of implementing missing (and undocumented) Redis Cluster commands.
I waited for users / developers opinion, but only @xetorthio replied it's better to implement all Redis commands.
And I've posted to Redis Google Group about stability of undocumented cluster commands, but @antirez didn't answer.

I agree @xetorthio's opinion and respect rule of thumb, so I've implemented all Redis Cluster undocumented commands currently not implemented.

All commands have tests except two things, CLUSTER SAVECONFIG.
CLUSTER SAVECONFIG just write config to config file, so I cannot make unit test.

Commands List
====

* CLUSTER FORGET
* CLUSTER FLUSHSLOTS
* CLUSTER KEYSLOT
* CLUSTER COUNTKEYSINSLOT
* CLUSTER SETSLOT STABLE
* CLUSTER SAVECONFIG
* CLUSTER REPLICATE
* CLUSTER SLAVES
* CLUSTER FAILOVER