Support configuration of serialization thru mix-in

```
It would be very nice to be able to use annotation-based configuration without
modifying the classes themselves. The only solution now is type converters,
which are much less convenient.

http://wiki.fasterxml.com/JacksonMixInAnnotations

I think being able to use a mix-in for (de)serializing a whole type hierarchy
would be a great addition and a big time-saver when dealing with "foreign"
classes.
```

Original issue reported on code.google.com by `floz...@gmail.com` on 10 Jun 2012 at 10:31