audio: fix volume property that wasnt working. closes #1311, closes #1269

Conflicts:
	kivy/core/audio/__init__.py
Fix relative import in audio_sdl.pyx. Sound now works on iOS!
(tito actualy fixed this, I just did the testing)
audio_sdl: Use Mix_Playing() instead of Mix_GetChunk() to detect if the sound has finished in ._check_play()
Add Looping support for Sound objects.
Tested with the GStreamer backend. pygame and SDL backends still need testing.
Forgot to commit the pygame backend
Because the pygame and sdl audio backends use Clock callbacks to check for when the sound finishes
care must be taken to avoid running .play() inside ._check_play(). Schedule it with Clock.schedule_once
audio: fix versionadded