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.
We should make that an optional parameter of this RDD. The DStream may set it depending on more semantic information abotu whether we are going to ever need this data every again. If we do, then it can be put in the BlockMAnager.