Merge pull request #1525 from dingpinglv/Iss3825_SimpleAudioEngine

Fixed #3825: fixed a bug of SimpleAudioEngineForMobile that the isMusicPlaying status is wrong
Fixed #3825: modify SimpleAudioEngine's _maxAudioInstance to 5
Merge branch 'develop' of https://github.com/cocos2d/cocos2d-html5 into Iss3825_SimpleAudioEngine