XBMC external audio track

外挂音轨，未被加到正式版中

参考：

• http://forum.videohelp.com/threads/362053-XBMC-13-Alpha-Added-ability-to-play-external-audio-track-with-videos
• http://forum.xbmc.org/showthread.php?tid=126156
• http://forum.xbmc.org/showthread.php?tid=165257
• http://forum.xbmc.org/showthread.php?tid=155415
• http://trac.xbmc.org/ticket/11446
• http://forum.xbmc.org/showthread.php?tid=155415
• https://github.com/xbmc/xbmc/pull/2562