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 expression says "I am ghcjs-dom-0.1.1.0", but apparently it's fetching a different version from Git than the one that was promised! This should not be done, really, the version number should accurately identify what it is that's being installed.

Besides, there's a newer version available from Hackage already, so I'm not sure why getting this from Git is necessary at all.