Release process

Adds a makefile target to build releases for different target architectures/OSes.

Also adds a script to upload the result as a github release.
The benefit is much less mental overhead - I know a lot of things about `make` that I don't know about a random, home-made bash script living in someone's Git repository. For instance, can I run it from the `scripts` directory, or does it have to be run from the root of the repository? I already know this about `make`.

I don't feel that strongly about it, though