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.
nit: this isnt quite exact, as if count % countPerBlock != 0, the number of elements created is > count; the last partition should have < countPerBlock elements (i.e. partial block), but this code creates full block instead of partial block. 

better code 

`(1 to count).grouped(countPerBlock).toSeq`