[enh] add facebook generic backend

You can do this test the other way arround using the hasattr buildin, something like this:

```python
if not hasattr(settings, "FACEBOOK_APP_ID") or not hasattr(settings, "FACEBOOK_APP_SECRET"):
    raise NameError("ERROR: %s disabled, please define FACEBOOK_APP_ID and FACEBOOK_APP_SECRET in your agenda settings file" % org_name")
```

This avoid one level of indentation and avoid possibly hiding another NameError that won't be this one thus making debugging harder.