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)
I did it and worked only with ```<%= form_for :race_variants, url: :race_variants do |f| %>``` because ```form_for :race_variants``` use the correct controller but the route is ```/races/:id/variants/:id``` and the ```resources``` used ```/races/:id/variants```.