now it can play/pause/resume/stop a particular effect
update isMusicPlaying() in cc.WebAudioEngine, now it won't return null when it's not playing
extract _pauseSound() and _resumeSound()
add pauseAllEffects() and resumeAllEffects(), but resumeAllEffects() will also resume those which have finished playing
finish stopAllEffects() and unloadEffect(); but there is a bug: effects of same resources cannot be played simultaneously at present...
add paused property in WebAudioSFX, now it cannot resume those which have finished playing