Run demo as commands

[benoitbryon]

At the moment, demo scripts are not really readable and poorly usable:

• they read environment variables, because that is something we can setup on travisci.org
• they prompt the user for missing arguments, but the prompt is quite raw

As commands, demo scripts will get an help. Users will be able to provide arguments with options such as demo/accounts.py --integrator-key="some-key".

[benoitbryon]

Warning: keep on using environment variables on travis-ci.org, because they are secure, i.e. do not reveal things like --integrator-key=some-secret in tox output.