Add subgid/row/cell components

(and removed the "react-addons" module, which is deprecated)
Use "real" futures module attributes

Each cell of type "module", will have, on their "content" node:
- component: the js path to the module to load
- *: all other attributes to ass

Also correct the Utils.count* methods in Grid components specs to handle
the "0" result.
Add two test modules
Attach/detach modules to cell when needed

A module is not directly rendered into its cell, to be able to easily
move it, for exampe in drag&drop mode, or when adding a new module.

Instead, they are rendered in "div" holders, not displayed on the dom,
and they are attached to the cell when the cell was just rendered, and
detached from the the cell when it will be rendered.

It avoids having to recompute the whole module when a cell changes.

This comit doesn't include any documentation or test for now, but it's a
work in progress (and it works, by the way)