#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).
Is it really useful to configure this per property? I feel it should be more an option on the mapper level and/or a processor option. Also I think it should be disabled by default as the normal case is to invoke the setter. If both a setter and an adder method are present and adders are enabled, I think we should use the setter as I feel that's what makes most sense in a mapping scenario.