#207 adder method for collections on target

For review, it is not yet completely finished. It contains TODO's in the `MapperCreationProcessor.java` and `Type.java`. It also lacks more unit test (placeholders are in place).
What is the behavior if there is an adder and a setter method for the collection at the same time? This should be described in the comment here. Thinking about it, it might make sense to even define it as follows to make the preferred behavior configurable:

    CollectionMappingStrategy collectionMappingStrategy();

with

    enum CollectionMappingStrategy {
        SETTER_ONLY,
        SETTER_PREFERRED, // use setter if setter and adder are present
        ADDER_PREFERRED;  // use adder if setter and adder are present
    }