Use constant bool instead of macro verbose flag

I read that it was a better practice to use a static const
private property than to use macro-defined constant.
soundsLoaded flag; separate playSound method

Wished to improve management of sound object memory
and avoid repeating code.

* new boolean soundsLoaded flag in WhichBillViewController
* soundsLoaded set to NO in viewDidLoad
* loading sounds is moved from viewDidLoad to viewDidAppear
* soundsLoaded set to NO on memory warning
* new method playSound:(NSString *)snd called whenever sound
should be played