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 almost sure we can merge the navigation with the user selected.

Something like:

```
<li class="races">
 <a href="/races">Races</a>
 <% if character_sheet[:race].present? %>
  #...
 <% end %>
</li>
```