added a token type to satisfy section 6.3.1 of OAuth 1.0a spec

When request token and token secret is exchanged for the access token, it cannot contain `oauth_callback` parameter, under current implementation that was being included. I added a new type `RSOAuthRequestAccessToken`. 
Now when you implement `fillTokenWithResponseBody:` with that type, it removes the access token from consequent request.

This is all done to satisfy Section 6.3.1 of OAuth 1.0a [spec](http://oauth.net/core/1.0a/#auth_step3).  No userspace code should be affected (as anything that worked before can continue to pass in `RSOAuthRequestToken`)