WIP: Make runCharmHook windows compatible

On windows, hooks (live every other executable) need an extension. This branch adds support for running powershell and batch scripts as hooks. For powershell, we need to pass in exta arguments to exec.Command()

This branch depends on https://github.com/juju/juju/pull/287
I think adding an OS field to version.Current that panics (as it does for "series") would serve better. So we could just do a version.Current.OS instead of all this. Will propose soon.