#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 either should omit JavaDoc on a method at all or write a more descriptive comment. Personally, I've come to use one of the following options:

* Omit it completely if the method is self-explanatory
* Only write a comment for the method itself to explain its purpose but omit @param, @return etc. if those are self-explanatory
* Do it completely (but e.g. not describe two parameters out of three)

IMO this makes for a more consistent and nicer experience when reading the JavaDocs, while focussing on those docs that actually add value. In the case at hand I'd probably go for the last option, as e.g. it's not 100% clear to me what `pluralPropName` is doing.