Call factory method as object method instead of class method

In my application particularly, I need to wrap any instance returned by factory method into decorator class which implements some common logic. This seems possible by overriding the `factory()` method of `PHP_CodeSniffer_Reporting` class. However, the fact that factory method is called statically from its own class makes impossible to override it in extended class.

Calling method as `$this->factory()` instead of `self::factory()` would make interpreter using the implementation declared in current class rather than in base one.