#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`.
Andreas brought up the same issue. I can imagine that such a meta annotation is useful when we have more types of custom annotations. Introducing it at that moment might be too late. However, if you introduce it, you must also enforce it. Do you see more applications of custom annotations coming up? Or did you have something different in mind?