Rename option :primary_key to :references for associations

Hmm... I think I got it all!

Hopefully the current coverage was enough because I haven't added any new tests -- I figured this wasn't supposed to add any new feature, just make things easier to maintain moving forward.

It sure was confusing to have :primary_key sometimes referencing the model's primary_key and sometimes referencing the primary_key option... which I am sure is the reason for issue #122 in the first place!