Added FlxSoundGroup for managing volume of music/sfx/speech etc separately, closes #362

I snuck another small modification in here, too. Just making a for loop more efficient.

`FlxSound` now has a group. You can set a sounds group either by adding the sound to the group, setting the group of the sound, or by passing the name of a group to `play()`, `load()` or `playMusic()`.

Presently groups must be added to the `FlxSoundFrontend` by calling `addGroup`.