Created a protocol for the wrapper to follow.
Removed config options that where just getters/setters to the underlying AVPlayer.
Made it possible to pass in custom AVPlayer that can be customized with these options.
Updated AVPlayerWrapperTests and added a couple tests for the rate property.
Added for SimpleAudioPlayer and QueuedAudioPlayer.
Made next and previous items in the queue non-optional, instead they will be empty when no items are in the queue.