#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`.
Perhaps a bit farfetched, but would you see situations were we might need 2 (or more) qualifiers to select the proper method? So, imagine we would have more `DateToString` mappers, with the same `@LongFormat` annotations on methods. It then might be useful to annotate the `DateToString` mapper itself, e.g. : `@AmericanDateNotation` and `@GermanDateNotation`. The mapping might then look something like:
```java
@Mapping( source = "productionDate", qualifiedBy = { @AmericanDateNotation.class, LongFormat.class } )
@Mapping( source = "firstScreeningDate", qualifiedBy = { @GermanDateNotation.class, ShortFormat.class } )
GermanMovies toGerman( ForeignMovies movies );
```