#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 think we should only consider English names for the time being. It's quite a corner case and I'd first wait for user feedback. We should consider just the most common special rules and maybe a way to configure special singular/plural combinations such as the mentioned child/children. This could e.g. be done via a simple resource file which is queried before applying the default rules. Users could provide/override that file if required.

> E.g. something along the lines of: useAdderMethodWithName

Yes, this would work but it would require explicit configuration for each occurence. A generic mechanism which maybe works for 90% of cases seems preferrable to me; For the last 10% users could configure exceptional cases as described above.