Rename reply action

[BrendanBall]

I think that the :reply action should be renamed since it only makes sense in handle_frame, but not in handle_cast and handle_info, where it is actually a bit confusing.
Additionally, :reply clashes with GenServer actions which I think is likely to be added in the future (Relates to #98).

I'm adding this here now because I'm adding a related callback in #112