#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).
To me it's just not that obvious whether an "add..." method should be invoked for a given mapping or not. Thus I think we should not use them by default but only if this feature has been enabled explicitly.

But as you say a project will either follow that style or it won't, so I think it makes sense to be able to enable it on a more global level then specific properties, i.e. per an attribute in `@Mapper` or even as processor option.

Or did you mean something else?