HDFS storage engine.

This is a first pass at providing `HDFS` support.  I use it to run a registry server on a `Mesos` cluster that currently lacks an `s3` storage layer.  Sadly it can't support more than one instance right now since it's not checking `HDFS` to update its `redis`/fs cache. Mainly because `hadoopy` is stunningly slow.

I'll change over all the reads to `snakebite` and then maintaining the cache might be a bit nicer. Not sure about those `redis` decorations. Most writes are immutable afaict (filenames are hashes of content) so they can be cached essentially forever w/o cache sync issues.  But the index file seems problematic.

Wonder if the storage layer api could be updated to note immutable content vs. content that might need locking to write and can't be cached?

Note: I'm using this with a 2.4 `Hadoop` cluster.