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
This final return feels awkward; we're returning something that will never exist, and then failing by coincidence. I think this function should return `(string, err)`, and do the `exec.LookPath`.