#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`.
> I think that's a good idea. There's only one small drawback: you might not have the custom mapper class under your control. But heck, in that case you can wrap it by another class that you have.

Yes, that's a good point. But wrapping should work as you say.

> I'll think a bit more about it and implement it as part of this one.

Cool.

> Ah. In that case: there are some more you should take a look at :smile:.

I know :-) I'm going to look at them one at a time.

> Perhaps a bit farfetched, but would you see situations were we might need 2 (or more) qualifiers to select the proper method?

Yes, I think that'd make sense. I suggested the `qualifiedBy()` specifically for that reason as it works with one or more qualifiers. Even if we'd allow only for one atm. we could relax it later on without breaking existing usages.