Adding powershell to chef shell-init

This partially replaces https://github.com/opscode/chef-dk/pull/187.
* chef shell-init supports powershell: `chef shell-init powershell`
* posh is aliased to powershell: `chef shell-init posh`

cc @btm @adamedx @opscode/client-engineers 
yeah we support that for install.sh, but for chef-dk we don't even support solaris and aix as workstations at all.