@ashfurrow => Lots of small changes

Polish:
* Move App State into something loaded by NSUserDefaults, meaning you can make changes in Admin that stay.
* Countdown runs exempt from main run loop, so scrolling works, and no lockups.
* I moved the callback for the switch view outside it's animation block. This has _mostly_ fixed that AutoLayout problem. Oddly if you leave it scrolling and hit the switch then we get animation issues. Perhaps before making any collectionview changes we need to ensure there's no inertia?
* Added a choose your Auction View Controller
* Clears cookies which get sneakily set meaning Auth would only work once per app session
* Can jump between registration states
* We found an odd state where someone would hit registration with an existing Artsy account, so we've given some instructions to make it clear that's OK
* Made a subtle animation for the Popover's appearance
* The bid preview price on the popovre was showing in pounds on my iPad, so to ensure it's consistent I force the locale to be US.
* The `A-Z` ordering is now done by the Artist, we have a sortable_id property just for this on the datamodel
* Minor UI polish