Return Promises from every stores.
Add simple GoogleMap component.

This component displays a map center at some location, and adds one marker at the center of the map.
Add a Post component, which can render posts of type "text" and "picture", for now.

See #18
Remove the Picture question component, as it is now a mere specialization of MultipleChoice.

See #18
Update React and react-async.
Switch from react-router-component to react-router.

Close #21
Display modal when attempting to quit the game.