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
You are correct, but in some cases, there may come the need not to make the charm itself open for some reason. In the windows world this is more commonplace then not.