ArcBees/GWTP — pull request #656 by rdwallis (2014-12-15 07:37:02+00:00)
Title: finalize ManualRevealCallback class

I'm an idiot but I often start off with an AsyncCallback and then convert it to a ManualReveal callback and end up with:

```
new ManualRevealCallback(aPresenter) {
@Override
public void onFailure(Throwable caught) {

}

@Override
public void onSuccess(T result) {
//do some work and then wonder why the presenter is never revealed.
}
}
```

This prevents the above by preventing ManualRevealCallback from being subclassed.