Deserializing to List<T> will return the list as items of first parent abstract type without warning

Either of these should be done to fix this trap:
 - Warn user that type information is lost.
 - Fix BinaryPack to return the type for every reference in the List<T> as it was serialized.

See #7 for more info and PR #10 that allows users to specify which parent type should be used when deserializing.