Store the user items selection on cookie

This PR is related to issue #20 and it's working in progress because I need create tests.

![fm-store-user-selection-on-cookie](https://cloud.githubusercontent.com/assets/1503773/5240038/d22f5c08-78e0-11e4-8211-df980d113047.gif)
Instead of use `post`, we can use the current `resources` helpers with `:create`.

For example: `resources :backgrounds, only: [:index, :show, :create]`