Apache spack off heap cache block manager over redis

See the following SparkConf settings for getting OFF_HEAP caching to work with redis:

conf = new SparkConf().
  set("spark.externalBlockStore.blockManager", "org.apache.spark.storage.RedisBlockManager").
  set("spark.redisBlockStore.url", "redis://localhost:6379").
  set("spark.redisBlockStore.keyPrefix", "spark")

This OFF_HEAP block manager enables caching RDD's in an external redis server. Data is cached in simple redis string values. Once set up you can persist your RDD's in redis like this:

myRdd.persist(StorageLevel.OFF_HEAP)

Tested against Spark v1.4.1 (make sure apache/spark#6702 is applied)