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/dlfp/feeds.py

@@ -21,7 +21,7 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 from logging import warning
 import feedparser
 import re
-import datetime
+from datetime import datetime
 
 class Article:
     RSS = None