#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).
> No. Dali has a simple set of rules. It therefor makes the wrong deductions on plural / singular form.

I'm still not clear when Dali is doing this deduction. In which situation does this happen?

> MapStruct now first tries to match up to these wrong deductions made by Dali (on the premise that this is the most used framework for entity-jpa code generation). 

Shoudn't we try to apply the correct form first? I don't think we really should assume the usage of any specific tool such as Dali, but I guess it doesn't do harm to do it as some kind of fallback.