Added AuthorizationResult

This is related to #180

I know that this PR creates some overhead, because the AuthorizationService actually implements already `getIdentity` but the permission's name as well as the identities roles are really really really important for many Assertions and since I do not want to break BC, I couldn't find another way. Also it's maybe not that useless, since we can now do:

```php
if($as->isGranted('something')){
   $result = $as->getAuthorizationResult();
   // Do something
}
```

This is explicitly written in that matter, that we do not need to tag 3.0