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 think we don't the `select` button on the races show. Because not make sense user click on `select` to choose the race when he need to click on `select` to choose the variant. When the user choose variant, automatically e chooses the race too.

We should remove this form on the races screen. Let's remove this form from this partial.

Let's put the form only on the the `races_variant/show.html.erb`. And add the the race slug as hidden field there.

Then, you don't need anymore the `params[:controller].to_sym`, you can use instead ` form_for :race_variants`