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.
I believe I moved this into the before_send so that the params and headers would be fetched already. I can try moving it up.