Add support for GHCJS

This provides a (AFAICT) fully working GHCJS package set.

@peti Can I get your feedback on how to best merge this in? I'm sure there are some things to consider, like if/how we should specify that a given package supports GHCJS (or more generically, any particular compiler).
No-one is going to configure anything like that we have no `aeson_0_8_0_0` in Nixpkgs. We have `aeson_0_8_0_2` since quite a while. In other words: these overrides to `null` accomplish nothing unless someone makes sure they always match our Haskell package set. Who is going to do that?