Add Makefile, adapt download script to python3 and add CI with automatic deployment

Adds Makefile to make it trivial to trigger download script during deployment. Download script itself had to be adapted for use with python3 (breaks compatibility with python2, but I think it would not be used anyhow). Finally adds CI to deploy package upon tagging.

@cburr, can you please have a quick look to cross-check all this.

Resolves #5 (closed)