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).
I'll take them out. While part of me wouldn't mind manually setting the correct versions, I imagine it would be easy for those versions to drift over time as we update our packages, which would be pretty confusing for users when their ghcjs breaks when updating to a new nixpkgs revision. Seems better to keep it simple.