Role Voter for closed user groups

this is somewhat similar to the state voter, but would be more explicit and straightforward to configure (or maybe its just some syntactic sugar around the state voter).

``` php
    RoleAccessVoter
    $role = $object->getRole();
    if (!$role) {
        return self::ACCESS_ABSTAIN;
    }
    return $context->isGranted($role) ? self::ACCESS_GRANTED :  self::ACCESS_DENIED;
```
