Add callback argument to track completion of dump

Oh, and another thing: `Persistent<T>` doesn't inherit from `Handle<T>` in v0.11 so you can't pass in `on_complete_callback` directly here.  You'd need something like e.g.:

```c++
Local<Function> callback =
  COMPAT(*on_complete_callback,
         Local<Function>::New(isolate, on_complete_callback));
```

You can obtain the isolate with `Isolate::GetCurrent()`.