Previous scene not rendered when pushing new scene

With the [current way I've written the rendering](https://github.com/jlyman/RN-NavigationExperimental-Redux-Example/blob/master/app/containers/AppContainer.js#L31) of `` the current scene is blanked out when pushing a new scene onto the stack, meaning the screen goes white while the new scene is animated on.

Haven't had a chance to look into a way of rendering out both at the same time (could be as simple as checking for >1 scenes and rendering both current and previous in stack). If anyone wants to fix this for us that'd be awesome.