Add an app with subjobs that download and upload files

Here's an app that I *think* should be able to work once the download/upload utilities change as I've described in the code review comments.  Note that it really wants to just be called with two input files.  Also, `dx run --watch` will be insufficient for your purposes for waiting until the job is done because it has subjobs.  You should add `--brief` to the call and strip the result to get the job ID, and then wait for that job to finish (e.g. with `dx wait job-xxxx` or with the dxpy utility for that).  Let's talk some more tomorrow if this doesn't make sense.