implement missing sound functions in gwt backend
reduce completion listener code in AndroidMusic
reuse IOSSounds, similar to Android's SoundPool
Properly dispose completion listener in openal
fixed completion listener firing when looping in gwt
Music completion listener uses postRunnable on mobile
Merge branch 'master' of https://github.com/libgdx/libgdx into gwtsound
adjusted access modifiers in GwtApplication
Merge pull request #498 from moly/gwtsound

Implemented missing sound functionality in GWT backend