Redis-py-cluster

* Pin upstream redis-py package to release 2.10.6 to avoid issues with incompatible version 3.0.0

* Add Redis 4 compatability fix to CLUSTER NODES command (See issue 217)
* Fixed bug with command "CLUSTER GETKEYSINSLOT" that was throwing exceptions
* Added new methods cluster_get_keys_in_slot() to client
* Fixed bug with `StrictRedisCluster.from_url` that was ignoring the `readonly_mode` parameter
* NodeManager will now ignore nodes showing cluster errors when initializing the cluster
* Fix bug where RedisCluster wouldn't refresh the cluster table when executing commands on specific nodes
* Add redis 5.0 to travis-ci tests
* Change default redis version from 3.0.7 to 4.0.10
* Increase accepted ranges of dependencies specefied in dev-requirements.txt
* Several major and minor documentation updates and tweaks
* Add example script "from_url_password_protected.py"
* command "CLUSTER GETKEYSINSLOT" is now returned as a list and not int
* Improve support for ssl connections
* Retry on Timeout errors when doing cluster discovery
* Added new error class "MasterDownError"
* Updated requirements for dependency of redis-py to latest version

* Package is now built as a wheel and source package when releases is built.
* Fixed issues with some key types in `NodeManager.keyslot()`.
* Add support for `PUBSUB` subcommands `CHANNELS`, `NUMSUB [arg] [args...]` and `NUMPAT`.
* Add method `set_result_callback(command, callback)` allowing the default reply callbacks to be changed, in the same way `set_response_callback(command, callback)` inherited from Redis-Py does for responses.
* Node manager now honors defined max_connections variable so connections that is emited from that class uses the same variable.
* Fixed a bug in cluster detection when running on python 3.x and decode_responses=False was used.
Data back from redis for cluster structure is now converted no matter what the data you want to set/get later is using.
* Add SSLClusterConnection for connecting over TLS/SSL to Redis Cluster
* Add new option to make the nodemanager to follow the cluster when nodes move around by avoiding to query the original list of startup nodes that was provided
when the client object was first created. This could make the client handle drifting clusters on for example AWS easier but there is a higher risk of the client talking to
the wrong group of nodes during split-brain event if the cluster is not consistent. This feature is EXPERIMENTAL and use it with care.

- Remove print statement that was faulty commited into release 1.3.2 that case logs to fill up with unwanted data.

- Fix a bug where from_url was not possible to use without passing in additional variables. Now it works as the same method from redis-py.
Note that the same rules that is currently in place for passing ip addresses/dns names into startup_nodes variable apply the same way through
the from_url method.
- Added options to skip full coverage check. This flag is useful when the CONFIG redis command is disabled by the server.
- Fixed a bug where method _CLUSTER SLOTS_ would break in newer redis versions where node id is included in the reponse. Method is not compatible with both old and new redis versions.

- Rebuilt broken method scan_iter. Previous tests was to small to detect the problem but is not corrected to work on a bigger dataset during the test of that method. (korvus81, Grokzen,
- Errors in pipeline that should be retried, like connection errors, moved, errors and ask errors now fall back to single operation logic in StrictRedisCluster.execute_command. (72squared).
- Moved reinitialize_steps and counter into nodemanager so it can be correctly counted across pipeline operations (72squared).