UPGRADE FROM 2.0 to 2.1

assets_base_urls and base_urls merging strategy has changed

Unlike most configuration blocks, successive values for assets_base_urls will overwrite each other instead of being merged. This behavior was chosen because developers will typically define base URL's for each environment. Given that most projects tend to inherit configurations (e.g. config_test.yml imports config_dev.yml) and/or share a common base configuration (i.e. config.yml), merging could yield a set of base URL's for multiple environments.
moved management of the locale from the Session class to the Request class

Configuring the default locale:

Before:
```
framework:
  session:
      default_locale: fr
```
After:
```
framework:
  default_locale: fr
```
Retrieving the locale from a Twig template:

Before: {{ app.request.session.locale }} or {{ app.session.locale }} After: {{ app.request.locale }}

Retrieving the locale from a PHP template:

Before: $view['session']->getLocale() After: $view['request']->getLocale()

Retrieving the locale from PHP code:

Before: $session->getLocale() After: $request->getLocale()