Refactor node ID handling, use check digits (fixes #269)

New node ID:s contain four Luhn check digits and are grouped
differently. Code uses NodeID type instead of string, so it's formatted
homogenously everywhere.

cmd/syncthing/gui.go

@@ -327,7 +327,7 @@ func restGetSystem(w http.ResponseWriter) {
 	runtime.ReadMemStats(&m)
 
 	res := make(map[string]interface{})
-	res["myID"] = myID
+	res["myID"] = myID.String()
 	res["goroutines"] = runtime.NumGoroutine()
 	res["alloc"] = m.Alloc
 	res["sys"] = m.Sys