Prevent fade from freezing with the main thread

[thecodewarrior]

Before this change the Timer would run on the main thread, meaning if your UI froze, so did the audio fade. Grand Central Dispatch allows it to run on a background thread as well run much more efficiently.

This simply converts AutoCancellingTimerInstance to use DispatchQueue instead of Timer.

[evgenyneu]

Hi @thecodewarrior, thanks for contributing. A couple of small issues I noticed

• Most importantly, the is a hiccup in the music when I launch the demo app. The sound is interrupted with a small period of silence.

• It shows "Main Thread Checker: UI API called on a background thread: -[UIView setHidden:]" error in the Xcode output window when I run the demo app.

• The "Fading out..." message in the demo app does not disappear immediately after the fade finishes.

• I think it will useful to write in the Readme that the finish closures are now called asynchronously.

Let me know if this can be addressed so we can merge this. :)