Deactivate workers for windows

Authenticationworker and rsyslog workers are not supported on windows yet. The authentication worker will be ported on windows to add support for winrm. Rsyslog is not supported on windows yet.

This pull request depends on https://github.com/juju/juju/pull/244
I find this to be confusing.... worker.SupportsOS really just is supportedOS.Contains(version.Current.OS) .. but sorta backwards.

I know you're trying to be forward-looking, but I think we can pretty safely just do  if version.Current.OS == Windows { // no-op worker }     And it'll be less confusing to read, IMO, and when we add other unixes, they'll be supported without a change here, which is good.