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 comment is now incorrect, unfortunately.  Probably better to just reference the slice above and not spell them out here, that way you don't have to update the comment to match the slice.