Proxy Implementation for Windows Chef Omnibus

As a corporate end user I need Chef Omnibus for Windows to work behind a proxy so I can be able to use it on my company with proxy enabled.
consider using:
```
  if (!$wc.Proxy.IsBypassed($chef_url))
  {
    $creds = [net.CredentialCache]::DefaultCredentials
    $proxyaddress = $wc.Proxy.GetProxy($chef_url).Authority
    $proxy = New-Object System.Net.WebProxy($proxyaddress)
    $proxy.credentials = $creds
    $wc.proxy = $proxy
  }
```

I dont have access to an actual proxy to test this but this is based on code I have used before and I remember needing to specify default creds. This may fail in a minority of cases where the proxy needs an alternate credentials specified. In chocolatey, for instance, we add this to account for that:
```
if ($creds -eq $null) {
      $cred = get-credential
      $creds = $cred.GetNetworkCredential();
    }
```

However, that would not work here since `get-credential` pops a dialog box on the guest. So we'd need a way of specifying the creds in a config. I don't think that is a MVP requirement.

Also, this accounts for cases where using a url that has been added to the bypassed urls in windows internet settings.