use the right python interpreter for cron job
use real quarterly STIG ZIP URL
Support continuing partial downloads.

We're trying to download and process a 180MB file and the cron job
only gets twenty minutes to run. Not even the download finishes in
twenty minutes. So instead of downloading to a TemporaryFile, now we
download to a well-known, content-based name, in a well-known
directory; if we have to quit, we can pick up the download again next
time.

Bad assumptions and incomplete code filed as issues.