Add windows userdata

This is a work-in-progress branch.

This branch adds userdata support for windows systems.
To be honest, in our particular use-case, I think we don't really need to. Permissions are inherited from the parent directory, and we set the permissions as soon as we create C:\juju. Come to think of it, we could check for runtime.GOOS and return the windows equivalent of writeFileCommands without actually needing a different function. Would that be ok with you guys?