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).
What kind of version number is this?

If possible, please use version numbers that increase monotonically, i.e. newer versions should have a bigger number. Otherwise, `nix-env -u` won't know when to update.