Support for star-like writable fields

[swistakm]

We need to define how this should be handled. The easiest approach is to assume that fields with source="*" argument should return dictionary that will be added to the object_dict instance. Anyway this will make sense only with custom serializer fields. So maybe it would be better completely remove "*" option from built-in parameters and maybe delegate set/get-attr/key responsibility to custom fields?

[swistakm]

This goes to 1.0.0 because requires backwards incompatible changes.