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 believe the @luite's changes have been merged into Cabal proper, so I think this can actually completely go away if I pull in the latest Cabal (assuming we don't already have it in master).