Implement the CocosDenshion Audio Engine via Web Audio API

Resend pull request, to "develop" branch this time.

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For browsers that support Web Audio API (Chrome, Safari on iOS 6, etc.), it will automatically pick the new cc.WebAudioEngine as Audio Engine.

The previous engine implementation uses \<audio\> tag in html5. Web Audio API is a solution to circumvent [some restrictions on iOS][restriction_link]: 

>the JavaScript play() and load() methods are also inactive until the user initiates playback, unless the play() or load() method is triggered by user action

Now, it can play multiple pieces of music and effects normally on iOS 6 and later. Only the first triggering user action is required for starting playing.

[restriction_link]: http://developer.apple.com/library/safari/documentation/AudioVideo/Conceptual/Using_HTML5_Audio_Video/Device-SpecificConsiderations/Device-SpecificConsiderations.html