use requests to download DEM

It's a little brittle to shell out to ``wget`` to download the DEM data when
Python has the wonderful [requests](http://docs.python-requests.org/) library.
Make use of it for downloading the DEM.

A secondary benefit is that the library can be mocked in the test suite meaning
that we don't need to *actually* have a full HTTP server stack up and running
in order to test the downloader.
Ah, yeah. Quite probably. I'll fix that in a separate PR after you've had a chance to review other things.