#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).
I'm wondering whether we should exclude null values from the source collection (i.e. only invoke add() if they are not null).

On the other hand the suggested behavior (i.e. without the null check) is consistent with the simple set() call we do atm., so this is probably the better choice. Maybe something to re-consider later on.