Simple compound keys

I'm sorry, I couldn't send feedback about PR #20. I agreed that recursive solution is not the best. But I spent time to see `Scope` implementation. I understood after reading your latest implementation. `RecordSet` is very good to read and your loop solution also better.
I fixed a little problem on `#find` method. I think that you intend to accept all compound primary keys at once. And because `#find` must return loaded object, raise missing attribute exception when passed arguments are not enough. (Over passed arguments are ignored, but explicitly raising exception may be better.)