Adding Elixir logger

RE - https://github.com/phoenixframework/phoenix/issues/235

@chrismccord I need to test this, as well as add the stacktrace to the logger.error, still but this is the basic start. Once https://github.com/elixir-lang/plug/pull/75 gets merged or something like it I can complete this.
Why before send? Shouldn't it be on call? I would move this to inside the controller (instead of leaving it in a plug) (since it only works inside the controller.