make isMusicPlaying() to be in cc.AudioEngine, less redundant
add extention checking when fetching data is required before playMusic()
now it can pause only when it's playing; it can resume only when it's paused
move isMusicPlaying() back to each implementation class, WebAudioEngine is not going to use cc.AudioEngine.isMusicPlaying property
now music's volume is saved in WebAudioEngine so it can be preserved among different music resources
use Web Audio API's playbackState property to indicate isMusicPlaying()
now that cc.AudioEngine.isMusicPlaying property is no longer used, _musicListener (called to update it when music is paused) can be fully removed in cc.WebAudioEngine