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.
I am not sure if the `try...finally` is really necessary. In this test code we dont expect this to ever fail. And if it every fails the test will fail and we have something bigger to worry about, not just dangling file descriptors.