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 
I'm confused. What I would expect to happen is:

```
chef shell-init posh | Invoke-Expression
```

Would do exactly the same thing as

```
chef shell-init powershell | Invoke-Expression
```

and both would be run in powershell, one just saves you some typing and possibly makes us seem cool to the powershell crowd :wink: