Initial implementation of HDFSBackedBlockRDD.

This RDD pulls the block from BlockManager, if it exists in the Block Manager. If it is not found, the data is read from a file on HDFS.
Cna you move this file to  `streaming/rdd/` and package o.a.s.streaming.rdd? Keeps things consistent with all the DStreams being defined in `streaming/dstream/`