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).
@peti I went with the `cabalJs` aproach; here's what it looks like: https://github.com/cstrahan/nixpkgs/commit/df55381b65faea5827314d5faa9cb47226e0ea3d. How does that look?