Ohai Powershell plugin

@adamedx @btm 
Here's a quick first go at detecting the powershell version. I still need to test it on other versions of powershell, but it seemed to work correctly when I told it to emulate an older version.
It's correct that powershell exists by default on win2k8r2 or later, but it can go down to 2k3. So we should just run the plug-in -- if there's no powershell, it should do the same thing as if there's no ruby, erlang, etc (same as other plugins). 
So the test can basically be :windows_only I think. FWIW, we have powershell installed on Chef CI systems down to win2k3.