#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).
Are you sure on this? I'm under the assumption that an 'adder' is there with a purpose. Most IDE's require manual coding for this (rather then generation of getter / setter method).

But if so, lets make it a processor option.