Record deferred instances

If a fixture references a property of a fixture defined in the same
file, instantiation of that object will be deferred until the referred
to object has been persisted. (The assumption is that the data you are
trying to reference is some sort of entity id).

After the first persist you could then run `Base::load()` to process
the incomplete instances to fill in the blanks and then you can
persist them again.

If you require a more stringent definition of what fields qualify you
may override `Base::refersToPersistedId($reference, $property, $data)`