Fixed a bug of cc.AudioEngine that it doesn't work on firefox after it compiled with advanced mode
Merge branch 'develop' of git://github.com/cocos2d/cocos2d-html5 into Iss5805_Async
Merge pull request #2108 from dingpinglv/Iss5805_Async

Fixed a bug of cc.AudioEngine that it doesn't work on firefox after it compiled with advanced mode