Enables System.volume on Flash Platform and for the WebAudioAPI. Still needs an implementation in HtmlSound.hx

Signed-off-by: Kipp Ashford <kipp@seven2.net>
System.volume now works with HtmlSound as well, fixing all of the sound types.

Signed-off-by: Kipp Ashford <kipp@seven2.net>
Merge pull request #66 from volkipp/global_volume

Enable a System.volume for easy global mute and unmute.