new system to load backends

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

weboob/backends/transilien/backend.py

@@ -25,6 +25,7 @@ from .browser import Transilien
 from .stations import STATIONS
 
 class TransilienBackend(Backend, ICapTravel):
+    NAME = 'transilien'
     MAINTAINER = u'Julien Hébert'
     EMAIL = 'juke@free.fr'
     VERSION = '1.0'