Added compression feature.

By setting `compression.enabled=true`, you can enable Gzip compression
for the target data.  Alternatively, you may override the codec by
  setting `compression.codec`.

@pgarbacki: This is a work in progress.  I'd appreciate your feedback.

I included some tweaks I made to get the end-to-end tests working, but I can drop those.  The gzip test also requires having the Hadoop native libraries installed (for gzip et al).

I did successfully run the end-to-end tests with both compressed and uncompressed data.
Rather than making this method universal, let's simply put the codec creation logic inside the suggested createCompressionCodec() method. There shouldn't be much code duplication.