fix the disappearing footer on the todomvc-flux sample

In fact this PR is to focus on some strange behaviour within react components.
The tag `footer` was not displayed. :-1: 

If you invert `<section id="todoapp" />` and the `footer` tags, the `footer` will be shown. :+1:
If you encapsulate the `<section id="todoapp" />` in a div -- as I did on this PR --, the `footer` will be shown. :+1: 

Otherwise it will not be. Some advice?