Remove deprecation warnings for new AR syntax
Remove singleton classes on CPK relations.

* Extract CPK methods to its own module and extending instead of opening the singleton class.

* Fix Marshal.dump. Makes caching work again.

This is similar to 169 but works with Rails 4.
Adding callback tests

This way we can be sure extracting CPK extensions into modules that we extend
the AR::Base instances with, doesn't break the callback chain.
Merge pull request #239 from nhocki/delegate_relations

Remove singleton classes on CPK relations