New Audio Engine[ios/android]

Design Specification: https://docs.google.com/a/cocos2d-x.org/document/d/1aQdB16BkKyLBFg-DFVqhrZ5VWsf70eY0bbArEOycDoM/edit

The completed parts:2d audio on ios/android.
std::string constructs an empty string, with a length of zero characters by default constructor.