#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'm wondering whether we should require qualifier annotation types to be meta-annotated with a special marker such as `@Qualifier`. It may not be needed right now but it may be helpful to recognize qualifiers unambigiously. I'm undecided, WDYT?