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 thought in a diferrent structure to store character sheets. Something like:

``` ruby
cookies [:character_sheet] = {
  background: background[:slug]
}

# Maybe we can user helpers like:
def character_sheet
  cookies [:character_sheet]
end
```