@ashfurrow => Use the RAC mapping to inline set 

Turns out there were a lot of issues around the RAC mapping. The worst being the compiler segfaulting. Trying to work around that eventually ended up turning JSONAble into a subclass instead. Downsides, as you know, are that we now have to cast straight away after a request.

This part of the mapping process is now tested :+1: 

The other aspect is that a SaleArtwork is now passed in to the popover. @1aurabrown - I'll note where I'm going from a model to changing view code using `RAC()`inline. It's not accurate ( as we get it in cents ) but I'll get around to that maybe in another commit or a new PR to show how to transform signal.
if so, then we bind the `currentLabel`'s `text`  to observe `saleArtwork`'s `openingBidCents` - it is then mapped from  number to a string. See Interpolation for [more info](https://developer.apple.com/library/ios/documentation/swift/conceptual/swift_programming_language/StringsAndCharacters.html).

 (note that this has to be `dynamic` - see `SaleArtwork.swift` line 29)