Option to get related resource_uri with polymorphic resources

Currently, when using polymorphic resources, we have `resource_uri`'s mapped according to the polymorphic resource's `resource_name`.
This is perfect and the good behaviour when we want to group multiple specific resources into a common "entity".
But in some cases (and especially when you use js frameworks like emberjs/data -our case- or backbone) it is useful to get the targetted object with its original (related) `resource_uri`. Finally, the polymorphic resource acts like a "transparent proxy".

Please, look at the tests and how I have implemented it.

If you're satisfied, I'll add some doc and more tests for this option.

Signed-off-by: Léo S. leo@naeka.fr