initial refactoring of how the Play tab view hierarchy is set up (#137)

change details:
- convert various controller classes into UIViewController subclasses
- use the UIViewController API for associating container and child view
  controller
- create new root view controller class PlayTabController
- refactor most of the stuff in PlayViewController into other classes
cleanup of how some initial view frames are calculated
separate iPhone and iPad view hierarchy into distinct view controllers

change details
- split PlayTabController into two device-specific subclasses
- place iPad-specific split view handling into left/right pane view
  controllers
reorganize files in "play" subfolder
fix board position stuff on iPhone
remove DebugPlayViewController
remove PlayView sharedView method
review coordinate label views' setup
more cleanup after view controller refactoring
fix status view observation of cross-hair changes
fix autorotation for iOS 5
fix iOS5 memory management (view purging)
update class docs and README.developer with the new view controller regime
BoardPositionToolbarController no longer creates objects unnecessarily on the iPad