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.
Shouldnt there be another test which involves when all the data is in HDFS ?
Also a nit: please order the tests from simple usecase to complex - all BM, all HDFS, partial BM, ...