Make Jedis Cluster more likely to antirez's redis-rb-cluster

JedisClusterCommand

* improvements on connection error handling
** if based on slot connection throws connection related exception, retry to random node
** if we retry with random node, but all nodes are unreachable, throw JedisConnectionException without retry
** try to release connection whether connection is broken or not

* bug fix : if asking flag is on, and success this time, set asking flag to off

JedisClusterConnectionHandler

* have flexibility on initializing slots cache
** allow some nodes connection failure - skip
** if current node is success initializing slots cache, skip other nodes
** if current node failed to initialize slots cache, discard all discovered nodes and slots

* set nodes if node does not exist in nodes
** it restricts JedisPool to replace - prevent IllegalStateException : Returned object not currently part of this pool

JedisSlotBasedConnectionGuaranteedConnectionHandler

* getConnection (random connection)
** check all connections by random sequence
** always return valid connection (able to ping-pong)
** throw exception if all connections are invalid

* some refactoring
CLUSTERDOWN : JedisClusterException
Fix bug in JedisCluster del command. Fix #568
Merge branch 'fix-jedis-cluster-failover-to-slave' of https://github.com/HeartSaVioR/jedis into HeartSaVioR-fix-jedis-cluster-failover-to-slave
Merge JedisSlotBasedConnectionGuaranteedConnectionHandler to JedisSlotBasedConnectionHandler