#168 allow to globally configure mappings using @GlobalMapping

I had to move ```@Mapper``` and ```@MapperConfig``` to ```core``` and ```core-jdk8```, as they now require ```@GlobalMapping```, which in turn requires ```@Mapping```... 
A slight variation of the above may be better:

    @MapperConfig(
        namedMappings = {
            @NamedMappingDefinition(
                name="abstractEntityMapping", 
                mappings = {
                    @Mapping(source = "id", target = "primaryKey") ),
                    @Mapping(target = "auditTrail", ignore = true)
                }
            )
        }
    )

And then:

    @NamedMapping("abstractEntityMapping")
    @Mapping(source="other", target="otherTarget)
    public CarDto carToDtoCar(Car car);