Support for related objects fetching

-Adds support for related objects fetching. In related objects (passed down as the sixth argument when fetching objects, or lists of objects) , Pipedrive API passes down things like pictures of persons, relevant user profile data etc that would be needed to properly render the underlying objects to a human being.
-Since related objects do not represent complete objects, they are not wrapped as collectionItems and thus have to be used as data stubs only. This is intentional by design.