Fixed: Return a Number from CPNumber>>numberWithBool:

The Cocoa signature for this method
```+ (NSNumber *)numberWithBool:(BOOL)value```
implies that a number is being returned. In Cappuccino a Boolean is returned.

This leads to some problems e.g. in ```addObserver:forKeyPath:options:context:``` with ```CPKeyValueObservingOptionPrior``` and ```observeValueForKeyPath:ofObject:change:context:``` with ```CPKeyValueChangeNotificationIsPriorKey```.

Setting ```CPKeyValueObservingOptionPrior``` leads to ```CPKeyValueChangeNotificationIsPriorKey``` being set to ```1``` (which should be equal to ```[NSNumber numberWithBool:YES]``` as documented in https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Protocols/NSKeyValueObserving_Protocol/Reference/Reference.html. The actual value though is ```true``` as ```[CPNumber numberWithBool:YES]``` does not return a number but a boolean. This commit fixes this issue.