Add locale parameter

Graph API has a functionality to get a user info in different locales.
On our project we prefe get a user info in `ru_RU` locale instead of default `en_US`. We added a parameter `locale` to `omniauth-facebook` for more flexibility.

Example:
```
config.omniauth :facebook, 'APP_ID', 'APP_SECRET',
                           { :scope   => 'user_likes',
                             :display => 'popup',
                             :locale  => 'ru_RU' }
```