[STREAMING] Add redis pub/sub streaming support

This patch adds native streaming support for [redis](http://redis.io) through the [Pub/Sub](http://redis.io/topics/pubsub) system.  When creating the streaming interface, the user specifies `channels` and `patterns` to watch.  The streaming RDD then receives all messages which match those channels and patterns in the redis database.

I have attempted to match the process used by the other external streaming plugins.  I have added test cases to verify correctness of this plugin.  I would appreciate any feedback required to have this merged into master.

Note, this patch uses the [rediscala](https://github.com/etaty/rediscala) library to communicate with redis, and thus uses the "akka 2.2" branch to match spark's Akka version and reduce conflict.

Additionally, I own all source code included in this patch and release all of the source code under the license of Apache Spark (Apache 2.0).