Added DebugMenu listener

Listener that handles both direct observation of notifications or can register an observer and selector via the DebugMenu interface
The notification name should not be the name of the key. It should be a string constant that is defined and accessible via the debug menu header:

```objective-c
// RZDebugMenu.h
OBJC_EXTERN NSString* const kRZDebugMenuSettingChangedNotification;

// RZDebugMenu.m
NSString* const kRZDebugMenuSettingChangedNotification = @"RZDebugMenuSettingChanged";
```

The notification's user info should contain a dictionary with they key : value that changed.