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)
Yes, because `cookie` is the global scope. If later we want to add other key with same name do store other kind data will not be possible and the things can get very confused.

That's why I prefer to scope these data on `character_sheet` key. Will be clearer to discern data from character_sheet between everything else.