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)
We can simplify this logic assuming the user can only select the `leaf` of races and variants. For example, user can't select only the Elf race. He must select the Elf variants. When he select one variant, the race slug should be sended together.