php code sniffs: allow accessing underscored property names

@rebuy-de/it-development 

We *do* use underscored property names all the time when working with Zend `TableRow` instances.
This will not change anytime soon.

I propose to remove the code sniff for underscores *when somebody is accessing* properties.

That is: 

* `$x = $bla->bl_ub;` will be allowed
* `private $bl_ub` will still be forbidden

edit: clarification