AEAudioFilePlayer: call alloc init on self in factory method

Currently an object of type `AEAudioFilePlayer` can be returned from the factory method, and this leads to  `unrecognized selector sent to instance` exceptions. 

This commit lets subclasses of `AEAudioFilePlayer` return an object of the subclass's type. 

See: 
http://qualitycoding.org/factory-method/