Use Mappers factory to instanciate used mappers annotated with @Mapper (with default component model)

When you specify a used mapper in ```@Mapper( uses = UsedMapper.class )``` and that ```UsedMapper``` is also a MapStruct generated mapper (i.e. it has an ```@Mapper``` annotation), then, for the default component model, it will not be instanciated with ```new UsedMapper();``` (which would fail), but using ```Mappers.getMapper( UsedMapper.class );```

I stumbled upon that while fixing a test for PR #67. This feature will be integration-tested in that test.