Provide a fake bank with a Weboob backend

This MR is a fix for #641 (closed). It builds on top of !445 (closed), which should then be merged first. I am opening this MR now just to remember I have this code, and so that everyone can already give it a try.

Left to do:

• Update scripts/dev-server.sh to auto-update on changes of the fake module.
• There is an offensive print in Weboob source code which makes the whole thing crash: https://git.weboob.org/weboob/devel/blob/master/weboob/core/repositories.py#L274. Not sure how to deal with it :/ (basically, stdout is no longer a valid JSON…)
• There is an issue with the fake module as VERSION in a Weboob module is an exact version compatible with this module, not a required minimal version. Hence, we should have such a module per available Weboob version out there :/ I have an idea to monkey patch it, but I'm not really happy about doing it…
• There are some errors remaining, especially when filling the customFields it seems.
• And the most important one, it lacks an icon! :p