add force flag to windows shutdown

Hopefully this addresses issue #7. More testing necessary before
closing.

sys_windows.go

@@ -39,7 +39,7 @@ func shutdownNow() error {
 	if err := checkExe("shutdown"); err != nil {
 		return err
 	}
-	err := exec.Command("shutdown", "/p").Run()
+	err := exec.Command("shutdown", "/f", "/p").Run()
 
 	//Not that this matters
 	return err