This is a new concept for many-to-many relationships, where the extra fields
from the middle table can be overlaid over the related model using the
AnnotatedModel paradigm. The overlaid fields are writable and updates are
saved to the middle model.

This is performed using a new ::wrap method of the AnnotatedModel class.

AnnotatedModel::wrap($what, $overlay, [$class])

Which will create a new class which is a subclass of `$what`, and will have
the extra properties in `$overlay` accessible as normal properties.
`$overlay` can be an array or a VerySimpleModel instance. The latter is
required for writable overlays.