Make the auth*_redirect methods all return Futures.

Make the auth*_redirect methods all return Futures.

The OAuth 1.0 authorize_redirect is asynchronous, but this is not obvious
since it doesn't take a callback and instead just calls self.finish.
This fails in coroutines because the request will be auto-finished too soon.
Add Future returns to this method, and to all the rest for consistency.

Update docs and add tests for the various styles of login handlers.