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).
The version has clearly gotten out of sync with the git version; furthermore, I don't think pulling from git meshes well with the versioning system too well regardless, since there's no monotonically increasing unique identifier available (as you mentioned on Cabal).  Originally, I pulled from git because the version of ghcjs-dom on hackage was out of date and not compatible with the version of ghcjs I was using.

I think we should change this to pull from hackage now.