#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`.
> 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?

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.

I'll think a bit more about it and implement it as part of this one.