Allow users to embed data a Request

I need to attach some data to every request, would you consider a small [change to the  Request interface](https://github.com/davidbirdsong/vulcan/commit/3dbdf89308f852f30e319e50d119a280a963cb74)?
This operation will crash if you haven't executed `SetUserData` before executing `GetUserData` as `userData` will bi nil, right?