message: Rendered NavigationMotion on navigate
  This tells the Native side that a navigation's happend and passes the scene number (the place in stack). It's up to the NavigationMotion module to add or remove activities so that they match the number of scenes/crumbs
diff: import React from 'react';
  +import {NativeModules} from 'react-native';
  import {NavigationContext} from 'navigation-react';
  class Scene extends React.Component {
  @@ -14,9 +15,22 @@ class Scene extends React.Component {
  var {state, crumbs} = navigationEvent.stateNavigator.stateContext;
  return (state && crumbs.length === crumb) ? {navigationEvent} : null;
  }
  + componentDidMount() {
  + var {crumb, navigationEvent: {stateNavigator}} = props;
  + if (!crumb)
  + stateNavigator.onNavigate(this.renderMotion);
  + }
  shouldComponentUpdate(props, state) {
  return state.navigationEvent === props.navigationEvent;
  }
  + componentWillUnmount() {
  + var {crumb, navigationEvent: {stateNavigator}} = props;
  + if (!crumb)
  + stateNavigator.offNavigate(this.renderMotion);
  + }
  + renderMotion(oldState, state, data, asyncData, {crumbs}) {
  + NativeModules.NavigationMotion.render(crumbs.length);
  + }
  render() {
  var {navigationEvent} = this.state;
  if (!navigationEvent) return null;
  