Fix Elm.cabal missing elm-runtime.js

Apologies in advance if you don't want people messing with your dev branch...
(I'm tracking your branch, but using cabal-dev which made running Build.hs awkward.)


Move Build.hs (which builds elm-runtime.js) into a cabal build step (in compiler folder)
The elm exec and runtime should never get out of sync
Having a separate Build.hs file also causes problems when using cabal-dev

Moving elm-runtime.js into the dist/ folder also shows that it's generated (and cabal clean will remove it)

Similar for types.json (although whether this should be part of the build step, I don't know)

Tested using cabal-dev, ghc 7.4.1