Improve JSX syntax for built-in containers

[aksonov]

Currently Router accepts only one type of children - Scene (actually it doesn't check element name at all now). It makes very complex structures hard to maintain and modify - you will see just numerous </Scene></Scene>...</Scene> - it is difficult to insert new scene into 'middle' of some element.

I propose to add new types for built-in containers - Stack, Tabs, Lightbox and use Scene for child only. It will allow to make JSX more user-friendly:

   <Stack key='root'>
        <Tabs key='tabbar'>
              <Stack key='tab1'>
                 <Scene key='tab1_1' component={Tab1_1}/>
                 <Scene key='tab1_2' component={Tab1_2}/>
              </Stack> 
              <Scene key='tab2' component={Tab2}/>
        </Tabs>
   </Stack>

[aksonov]

Also I made key optional.