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.
Is it not the case that the correct behaviour of this line relies on an implementation detail of CPython, namely, that the `open(tif_path, ...)` will have its ref count drop to zero and be flushed & closed?