use cElementTree and elementtidy as default encoders, and added wrappers to ElementParser and HTMLParser if they are missing

Now there is a single file ~/.weboob/backends, where every backends are
instancied. There are two ways to create backends for frontends:
- Use Weboob.load_backends(), to load every backends in the config file
- Use Weboob.load_modules(), to instanciate every module one time