Remove singleton classes on CPK relations against Rails 4.1

This is similar to #239 but targets AR 4.1 instead of 4.0.

[We Heart It](weheartit.com) is using #239 in production right now and everything is Ok for us.

We're having the same `Marshal` problems on the Rails 4.1 update, so I'm submitting the "same" PR for 4.1.

I can cherry-pick the callback tests added on #239 once they get merged & accepted.