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 am pretty sure heredocs is going to insert extra new lines. You probably want to wrap it in a function too, so we don't generate the whole string for production when we won't even use it. Like `Logger.debug fn -> ... end`.