Fixes memory allocation crash with iOS 7 on device

There is an issue in iOS 7 when running on device whereby the non-utilized view controller (album or asset) is deallocated twice.  This fixes the crash and also simplifies the code.

Rather than create both controllers (album and asset) then removing the one that is unused, just create the appropriate controller and skip the other.  For the proper lifecycle see:

http://www.apeth.com/iOSBook/ch19.html#_container_view_controllers