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).
My memory is a little vague, but I think this had to do with OS X compatibility; it absolutely is overkill, and we should find a better approach.