Used UserStorage delegation instead of inheritance

Changed UserStorage inheritance to delegation for case when custom (basic) UserStorage is used.
To set custom parent UserStorage is necessary to set defaultUserStorage service in doctrine2identity namespace. When no defaultUserStorage is defined, default Nette\Http\UserStorage is used. 

```
doctrine2identity:
	services:
		defaultUserStorage:
			class: FooNamespace\FooUserStorage
```