Add Windows Support to Client DNS

[emmettmcdow]

At the moment we have no way to run the client in a Windows environment. Lets start by porting the client/dns module to Windows.

You can do this by making a dns_windows.go file and implementing the same interface functions as you see in dns_linux.go and dns_darwin.go. Basically you need to be able to configure the system DNS to point at a specified IP.

This needs to pass the existing tests in the client/dns directory.