message: Set title even if no navigationEvent in state
  If fluently navigate 2 scenes then still need the title set on the previous scene for the back button text on ios. Have to take this from the navigationEven prop instead of the state
diff: @@ -88,10 +88,10 @@ class Scene extends React.Component<SceneProps, SceneState> {
  }
  render() {
  var {navigationEvent} = this.state;
  - if (navigationEvent) {
  - var {state, data, title} = navigationEvent.stateNavigator.stateContext;
  + var {crumb, navigationEvent: {stateNavigator}} = this.props;
  + var {crumbs, nextCrumb} = stateNavigator.stateContext;
  + var {state, data, title} = (crumb < crumbs.length) ? crumbs[crumb] : nextCrumb;
  title = (state.getTitle && state.getTitle(data)) || title;
  - }
  return (
  <NVScene
  title={title}
  