Build a URL to create an OAuth dialog.

def get_auth_url(self, app_id, canvas_url, perms=None, **kwargs):
        """Build a URL to create an OAuth dialog."""
        url = "{0}{1}/{2}".format(
            FACEBOOK_WWW_URL, self.version, FACEBOOK_OAUTH_DIALOG_PATH
        )

        args = {"client_id": app_id, "redirect_uri": canvas_url}
        if perms:
            args["scope"] = ",".join(perms)
        args.update(kwargs)
        return url + urlencode(args)