Support rsync and/or aspera as an alternative download method

[kblin]

As described in some comments on issue #15, there is some interest in allowing downloads from NCBI to go via rsync or Aspera's ascp tool.

[andrewsanchez]

It seems like the solution here would be to have ncbigd generate some output that rsync or ascp can take as input. Probably less complicated and more flexible than calling rsync with subprocess.

[kblin]

I don't know (or care much) about aspera, but I know that rsync supports an --include-from directive that I think just is a file of paths to grab. We could generate that, I guess.
That said, rsync will not be faster than what ncbi-genome-download can already do, or even slower if you can run parallel downloads, as rsync is single-threaded.