Refactor/debug fullscreen

Using https://github.com/elm-lang/Elm/pull/746, we can get the debugger object from the handle to the main elm instance. This is compared to the previous method where we waited for the debugger to post to the main window that it is ready, then get the debugger object from `Elm.Debugger`.

This is a step towards better code and being able to have several debugged instances of elm on one page.

There should be fewer commit to look at once https://github.com/elm-lang/elm-server/pull/19 gets merged in.