add public interface for sharedManager

STKAudioPlayer.h

Follow the the singleton pattern:
To access audioPlayer instance throughout the app, without having to pass the data around manually.

Solve the problem: https://github.com/tumtumtum/StreamingKit/issues/110

Access audioPlayer instance globally by using:
(for example)
[[STKAudioPlayer sharedManager] pause];

I think it's good for StreamingKit to have this feature.