Made  property accessible in subclasses

[The documentation](https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Resources/doc/overriding_forms.md) encourages subclassing the existing forms and this makes life a lot easier. However, if you want to change the default options of a class, for example to add `cascade_validation`, you need to reimplement the `$class` variable and the `__construct()` method. Just because `$class` is private and used in `setDefaultOptions()`.

I made it protected so you can re-use the property when subclassing.