Add related_to query constraint.

This adds a related_to method to Query which can be used to query objects that are in a relation in another object.

I tested this within a project I'm working on that has object with relations.  I would have liked to add tests to parse_resource how to do so given that I am not sure how to create the relations as part of the test.  If there's a way to do that I'd be happy to try to add the appropriate tests.