Some utility/convenience methods added to jPlayerPlaylist prototype to:
1. get current track
2. get current artist
3. get playlist length
4. get playlist completion rate
jPlayerPlaylist is passed callback functions to be called on:
1. song change (ended or skipped)
2. song end (song played till the end)
3. song skip
4. playlist end
jPlayerPlaylist differentiates between song end and song skip
jPlayerPlaylist measures time spent on each track (it is passed as a param to song related callbacks)
jPlayerPlaylist shows artist name alongside the track name.

NOTE: we assume that playlist songs contain related artist info