Update SoundManager.as

Hi, I have recently been adapting this class to my Starling game and I thought I would share the few bits I have been able to simplify and a few new methods to deal with gradual volume changes.

**CHANGES:**
I added a 'soundIsPlaying' method to encapsulate the check for a playing sound that happened in various methods.

Also I added a couple of new methods to tween sound's volume: 'tweenVolume' and 'crossFade'. The first uses Starling's tween engine, but the tweening part can be performed by any tweening engine (i.e. TweenMax or TweenLite)- I am not too familiar with the Citrus Engine so I am not sure which one Citrus supports.

Hope it helps!