download: reduce reliance on shelling out

Remove reliance on wget and unzip being installed by making use of
Python's amazingness. Specifically we use the requests library to
download the DEM in-memory and then extract the TIFF to disk. This does
require using ~50MiB of RAM in the downloader but, hey, memory is cheap
nowadays, right?
Merge pull request #17 from rjw57/download-using-requests

use requests to download DEM