OHAI-491: Implement root_group for Windows 

Due to Windows-only bug [OHAI-490](https://tickets.opscode.com/browse/OHAI-490), the root_group plug-in was disabled for Windows. This change restores the functionality, and uses a purely local SID translation operation to avoid the issue we hit in the earlier implementation based on an inefficient WMI provider for the Win32_Group class. The LookupAccountSid method used in this implementation does a purely local lookup on one particular well-known SID, the one for the administrators group, and translates it to a name.

So the number of groups defined by the local system is not relevant, and certainly groups defined in AD will have no impact on the performance -- it's a simple local lookup of one SID.
It's a lot less good than including the dependency in the gemspec, because that's what bundler, etc. look at when making their dependency graphs. But if you wanted to do it, you'd do:

```ruby
begin
  gem "poo", "~> 1.5.0"
  require "poo"
rescue LoadError # In practice this is Gem::LoadError which subclasses LoadError
  # do something else
end
```