add xbox example and make some changes to support it

usb/debug.go

@@ -0,0 +1,22 @@
+package usb
+
+// To enable internal debugging:
+//   -ldflags "-X github.com/kylelemons/gousb/usb.debugInternal true"
+
+import (
+	"io"
+	"io/ioutil"
+	"log" // TODO(kevlar): make a logger
+	"os"
+)
+
+var debug *log.Logger
+var debugInternal string
+
+func init() {
+	var out io.Writer = ioutil.Discard
+	if debugInternal != "" {
+		out = os.Stderr
+	}
+	debug = log.New(out, "usb", log.LstdFlags|log.Lshortfile)
+}