Refactor/layers

- All layers added except the forestChange layers and stories layer.

- CartoDB layers are added to the map as the others (with mapoverlay), so we can go back to have the add/remove layer on MapView.

- All layer class have a getLayer() method which return a deferred object with the layer (this works nice with cartodb layers).

- No tests here, i just changed some views.
maybe just add a quick doc here for why we're using deferred promise.