provide field mapping for superclasses

A driver must be able to provide a field mapping for a superclass to
another driver. Here the problem was that the annotation driver was
relying on the reflection to get the mapping for a parent class, making
the wrong assumption that the parent class was mapped with annotations,
and making the wrong assumption that children classes are also mapped
with annotations.

fixes #517
This formatting seems a bit strange to me, maybe something like the following:

````php
if (
    $metadata->isInheritedField($property->name)
    && $metadata->name !== $property->getDeclaringClass()->getName()
) {
    continue;
}
````