Update README with `$this` information
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)`
Revert $this

This reverts commits bc9ba84b6b555e6ca1f690e7a9cbeb20b42517cc and
aadd5c36e6dfae038bd0aefab0d539d7d7431ac0.