- remove support for layered sounds
- refactor/clean up volume controls
- actually make it work
Adds a check for the masterVolume being undefined
- fixed the initialVolume setting.
Rewrote the sound group functionality.  There is now a first-class
object for each sound group with a lot more built-in code.

This may have finally fixed the bug where we get multiple VO playing
at the same time.  I hope.  However, it may have introduced a new bug
where VO doesn't play.  I've left a fair amount of debug logging in.
If VO is missing (or if you see any error messages), please save the
log and send it to me.
Merge pull request #312 from virtual-world-framework/sound-driver

- fixed the initialVolume setting.
The PlayingInstance now keeps track of its state, including
asynchronous states like "stopping."  This is used to better
handle race conditions and edge cases.

This should finally clear up the sound bugs - except for the
ones I haven't found yet! :)
Merge pull request #316 from virtual-world-framework/sound-driver

More sound driver work.
Merge pull request #317 from virtual-world-framework/mars-game-staging

Mars game staging