This includes updates to elm -compiler, -make, -package and -reactor.
-repl's test suite currently breaks.
The build for elm-reactor could be more concise, but it's clean. The
final build step for elm-reactor ultimately relies on elm-package
downloading some libraries, so anything radically better would probably
need nix to step in and somehow emulate elm-package. That would take
more knowledge about elm & nix than I have...
Note that elm has a different package split: The old Elm 0.13 package
is now elm-compiler, elm-make and elm-package.
Instead of invoking "elm" one now has to use "elm-make".
I kept the 0.13 version of elm around in case someone depends
on it.
