#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).
> The for-loop is still driven by the accessors, so wouldn't we miss the case where whe have no setter nor getter, but still an add method? I think we wouldn't make use of this right now. Admittedly having no getter may seem a bit contrived, but still it's a valid use case IMO.
> 

How can you be sure that the underlying data type is a Collection? We can only guess by the fact its an `add` method. Is that case strong enough?

> I think we should first create a list with all target accessors of the given type, be it setters, getters or adders - resolved as per the applying configuration - and then iterate over that list.

I thought about that initially. But you need to have a getter / setter to establish the type parameter in order to look for an adder method. I guess it could be done more concentrated i one spot though.