message: Named back stacks by crumb
  So can pop back to specific crumb
diff: @@ -103,7 +103,7 @@ public class NavigationStackView extends ViewGroup {
  int currentCrumb = oldCrumb;
  if (crumb < currentCrumb) {
  FragmentManager fragmentManager = ((FragmentActivity) mainActivity).getSupportFragmentManager();
  - fragmentManager.popBackStack();
  + fragmentManager.popBackStack(String.valueOf(crumb), 0);
  }
  if (crumb > currentCrumb) {
  FragmentManager fragmentManager = ((FragmentActivity) mainActivity).getSupportFragmentManager();
  @@ -113,7 +113,7 @@ public class NavigationStackView extends ViewGroup {
  String key = keys.getString(nextCrumb);
  fragmentTransaction
  .add(this.getId(), new SceneFragment(key))
  - .addToBackStack(null);
  + .addToBackStack(String.valueOf(nextCrumb));
  }
  fragmentTransaction.commit();
  }
  