Backwards compatibility and enabling unit tests for AudioRepository

[gregor]

Turns out the bug in Karma was new in the latest release already filed and fixed:
karma-runner/karma#2310

I also added backwards compatibility for Firefox for the non Promise based Audio.play()

[herrmannplatz]

Why not using a single promise chain like:

Promise.resolve()
.then ->
  if not audio_id or not audio_element
    throw new z.audio.AudioError 'Missing AudioElement or ID', z.audio.AudioError::TYPE.NOT_FOUND
.then ->
  if audio_element.paused
    audio_element.loop = play_in_loop
    audio_element.currentTime = 0 if audio_element.currentTime isnt 0
    return audio_element.play()
  else
    throw new z.audio.AudioError 'Sound is already playing', z.audio.AudioError::TYPE.ALREADY_PLAYING
.catch (error) ->
  throw new z.audio.AudioError error.message, z.audio.AudioError::TYPE.FAILED_TO_PLAY
.then ->
  @currently_looping[audio_id] = audio_id if play_in_loop
  return audio_element