Use Hadoop to run source dataset downloads in parallel

[tomwhite]

DownloadDatasetParallelTask in dag.py handles the parallelization of the source downloads itself. It would be preferable to let Hadoop do this for a couple of reasons: 1) it removes the SSH dependency (this will allow Eggo to run on a non-fabric provisioned cluster), and 2) fault tolerance is handled by Hadoop rather than Eggo code.

The simplest way is probably to write a streaming script that uses NLineInputFormat so each mapper can download a single source file.