[WIP] @1aurabrowb => First pass at sync

So @orta and I had talked about using [DRBOperationTree](https://github.com/dstnbrkr/DRBOperationTree/blob/master/DRBOperationTree/DRBOperationTree.m), which I think would be the best long-term solution. For now, syncing should be as simple as getting the latest auction listings and updating the existing models, which is what this does now. The cells themselves are

The ReactiveCocoa stuff is pretty advanced, so let me where you have questions. 

What's missing is: 

- [ ] *Not* syncing whilst a modal is presented (we don't want a bid that was valid in the UI to all of a sudden become invalid – let them find out after they try and submit is what we decided on IIRC)
- [ ] Testing (this is gonna be fun)
Yeah, there's *one* edge case I can think of: where an artwork has been added and another one removed. The count is the same, but the two arrays no longer "line up". I'll be a bit more defensive.