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/frontends/dummy/scripts/dummy

@@ -28,7 +28,7 @@ class Application(BaseApplication):
     APPNAME = 'dummy'
 
     def main(self, argv):
-        self.weboob.load_modules()
+        self.weboob.load_backends()
 
         for name, backend in self.weboob.iter_backends():
             print '= Processing backend name = %s' % name