Downloaded data files stored in /tmp directory.

Issue #4. Keep things a bit more tidy.
Config file now named 'config.json'

Issue #2.
Cleanup, work in progress, setup.py

Decided to add setup.py, turning this into a distribution, get rid of
the need for relative imports and / or PYTHONPATH stupidity.
Issue #1 (add 'mapping' to config)

But really much more, ongoing and chaotic cleanup of the code base.