Make 2 "Background Music" tracks that allow the dev to start and stop tracks

[TheNotary]

See issue #1

Make it so the user can

context.bgTrack.downloadAvailable("long_sound1.mp3", "bg1")
context.bgTrack.downloadAvailable("long_sound2.mp3", "bg2")
context.bgTrack.main.load("bg1")
context.bgTrack.main.play()
context.bgTrack.main.pause()
context.bgTrack.main.play()   // continue from where you left off
context.btTrack.main  // returns "bg1" 

context.bgTrack.crossFade(bg2)   // switches bgTrack.main from bg1 to bg2
context.bgTrack.switch(bg2)      // Stops playback of bg1 and begins playback of bg2

context.bgTrack.alternate.load("bg3")
context.bgTrack.alternate.play()   // play a second BG file at the same time as another