Allow :parent and :model options to be specified together

[shaug]

This is the same underlying problem as the #183, but I don't think the model can always reliably be inferred from scope, so I didn't want to vote it for fear it might be further dismissed, even if the underlying issue is valid and solvable.

It is possible to specify a has_many with a class_name on a parent class, which is necessary when the name of the resulting accessor does not automatically map to the correct class name for the model.

As such, it seems decent_exposure should allow parent and model to both be specified. When the above configuration is used with the current code, the parent short-cut cannot be used (even though it would seem natural), because the model method in behavior.rb fails to automatically determine the correct model name.