Fix: check volume upper & lower limit

Limit HTMLAudio, HTMLVideo volume to prevent unintentional error like
using out of range value
Fix: remove old webkitAudioContext

playbackState had been removed from standard AudioContext

https://developer.mozilla.org/en-US/docs/Web/API/Web_Audio_API/Porting_webkitAudioContext_code_to_standards_based_AudioContext
Fix: add ctx.decodeAudioData result checker

IOS(6/7) bug: if the user invoke play before ctx.decodeAudioData finish.
This would cause ctx.decodeAudioData to be fail silently without calling the error callback.