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
Can we add .exe to this?  I don't know that we'll have compiled hooks, but it certainly seems like it should be supported.