#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`.
Btw. didn't you like my original proposal of `qualifiedBy()`? I like it better because it reads nicely with one as well as with several qualifiers being given.