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)
Is it really work?
I tried put this and I had a "No route matches" because when you put in ```resources``` helper Rails create the routes like this:
```ruby
POST  /backgrounds  backgrounds#create
```
and I need something like:
```ruby
POST  /backgrounds/:id  backgrounds#create
```
Do you know if have another type of configuration?