Better `demo.py`

[thomas-riccardi]

• proper logging
• log start and end for easier log human parsing
• don't hardcode /tmp
• better packaging (see Publish on pypi #31 : do we want to install it via pypi as part of the lib? or as sample/example somewhere in the package)

[thomas-riccardi]

[ ] log debug to get wait() GET /task retry traces at least on automatic tests

[maingoh]

I am not in favor of including the demo in the package, I don't know any package that contains samples.
I think a better readme/documentation eventually linking to the demo.py (outside the package sources).