Composite idAttributes

Beginning a branch to address #86 - composite `idAttribute` keys on models.

So far, I've only got a rough implementation of the base model. I still need to figure out how this will play into the relations, but I figured if anyone has ideas or opinions they could chime in here.

/cc @brianjmiller - I think you were the one mentioning looking for this feature, do you have any thoughts on what the api should look like when forming relations on models that have multiple column pk's?