First draft of object model chapter

As of right now, I find myself wondering why we even bother having `fields` as a parameter at all, because it seems like we never use it (always pass empty.) Maybe a test showing how these are used would make it seem less extraneous. 

Also, at this stage it seems like objects are tracking their base class for no reason at all, because it's not being used for anything. I assume that the reason for this is coming up soon, but maybe a warning that we're doing some stuff 'in advance' here might help.