Change TypeMarshaller::unmarshal() signature

This pull request changes `webservices.rest.TypeMarshaller::unmarshal()`'s signature from `unmarshal(var $in)` to `unmarshal(Type $target, var $in)`.
...target, var $in).

This way, type marshallers know to which exact type to unmarshal to instead of having to use the base type, and thus makes unmarshallers reusable!