Add a Makefile, which allows for easy `make test`

This makes a stab at providing a stub for a Makefile, currently only for providing `make test`.

It automatically sets up a virtualenv in `build/virtualenv` and honors `DJANGO_SETTINGS_MODULE` from the environment.

I think it makes a lot of sense for a project to provide an easy way to run the tests, and nothing beats `make test`.

Not having `make` available by default on some systems (e.g. on Windows) should not prevent us from providing an easy entry point.