#268 introduction of qualifier

The solution is more-or less ready.

I need to add some additional tests to cover different kind of qualifying situations such as:
1. class
2. #method

Also I need to add test cases for the `@MapMapping` and `@IterableMapping`.
My original idea for the qualifier approach was a bit more indirect. What I had in mind is that one qualifies the role of a mapper (or mapping method) via custom annotations and the specifies the required qualifier(s) at the usage site by referring to that qualifier type.

E.g. like so:

    @Mapper
    @Subtitles // custom annotation type, maybe tagged with a marker such @Qualifier
    public class SubtitleMapper {
    }

And then:

    @Mapping( source = "englishTitle", target = "title1", qualifier = Subtitles.class )
    GermanMovies toGerman( ForeignMovies movies );

This avoids referencing mappers/methods by names and thus e.g. facilitates name changes. Also one can change the mapper satisfying a given role to another implementation, without affecting all its usages. So the qualifier really is an alias. WDYT?