Added scope to zone

This PR refers to this RFC: https://github.com/Sylius/Sylius/issues/1582
Scope has been added to zones, a service is used to retrieve the scope choices, if no service is defined, a default one is created and gets the choices from configuration:
```
sylius_addressing:
    scopes:
       shipping: Shipping
        content: Content
        price: Price
```
or
```
sylius_addressing:
    scopes:
        - shipping
        - content
        - price
```
I've created a twig extension so that the choice value is displayed nicely in the list view, but I'm not sure if this is the correct way to do it.

Also if no scope is defined in configuration, a select with a blank choice is displayed, should we display a 'Default' scope instead?

Behat tests and validation are still missing, but I'd like to confirm that what I've done is correct before moving on.
 
![screen shot 2014-06-09 at 16 05 37](https://cloud.githubusercontent.com/assets/4030438/3218203/f1f3bd72-efe7-11e3-9a8d-478b3d4c1fba.jpg)


This field must be nullable.