Added an option to get related resource_uri with polymorphic resources
Includes basic tests
Use an additional mixin for get_related_resource
Changed meta property name to prefer_polymorphic_resource_uri
Added a few documentation
Check if resource is registered or don't override resource_name.
Added some tests for this purpose
Merge pull request #38 from leo-naeka/related_resource_name

Option to get related resource_uri with polymorphic resources