Pass relation as second argument to relatedModel function.

For one example of where this is useful, please see:

https://gist.github.com/monokrome/5954751

This happens in any case where a function is abstracted out and the relation data can not be assumed, so I would think that it would be a fairly common issue.
Verify that the expected relation is passed.
Modified test for #50 to represent a usage example.
Removed a sneaky tab.
Extracted key string into a variable.
Merge pull request #50 from monokrome/master

Provide the relation itself to relatedModel function handlers.