Yeah, that sounds good to me.  I'm not sure you can completely separate it because you need to deal with header auth.  But I think an improvement would be to just have it be a variable that's cons'd up here, passed into/returned from `_set_oauth_params()`, `_parse_optional_params()`, `self.signature.sign()`, and `_get_auth_header()`.

The only argument for doing it now (rather than in a 0.5.x release later) is that the signature objects are technically part of the API and someone could implement others if they wanted.