Debugger shell working as a separate shell

* Added IEx.Debugger.Helpers, providing a copy of the helpers on
  IEx.Helpers with some extra debug functions.
* Debugger shell has functionality to run code on arbitrary nodes,
  but some aspects need to be thought through:
  - How a user should specify a pid?
  - How output from a previous command should be presented?
* TODO: The Evaluator from the debug shell will always run commands
  in background, so new commands can be issued while others are
  running.