Make it easy to specify on the command line:
- which python version to use (`make python=<version>`), where `<version>` is one of 23, 24, 25, 26 or any other key defined in the Makefile. If no version is specified, this defaults to the python program currently in the PATH, as before.
- which db backend to use (`make db=<backend>`), where `<backend>` is one of postgres or mysql. If no backend is explicitly specified, the database backend defaults to the SQLite :memory: kind.
The setup for Python and database backends is for now placed in the Makefile itself, but I'll move that in a second step in a Makefile.cfg file.
