Adding powershell to chef shell-init

This partially replaces https://github.com/opscode/chef-dk/pull/187.
This needs to be placed on top of https://github.com/opscode/chef-dk/pull/254
:construction:

cc @btm @adamedx @opscode/client-engineers 
If we're allowing 'posh' as a synonym for 'powershell', we should add it here, too.