Exposed public methods for setting the current playing item and exposed ...

...the queue (readonly) for anyone using the library to be able to get a better look at what's in there.

Basically I didn't change any of the methods except by adding new things.

* Exposed the queue, readonly.
* Added `playItemAtIndex:` and `playItem:`
* Added a boolean property (set to YES by default) to allow people to choose whether or not skipping backwards defaults to skipping to the beginning of the track.