Support abstract complexTypes - abstract modifier generating

In depend of issue https://github.com/wsdl2phpgenerator/wsdl2phpgenerator/issues/123 I've added a abstract modifier support in this commit, tests included. Hope, you'll agree.
I would prefer to have a setter for this as we have for [base type](https://github.com/honzap/wsdl2phpgenerator/blob/add_abstract_modifier/src/Wsdl2PhpGenerator/ComplexType.php#L164).