Updated WP8 prebuilt libs and dlls. Added Universal App prebuilt libs and dlls

This pull request updates the external dependencies for building the Windows Phone 8.0 (WP8) and the Universal App (Windows Phone 8.1 and Windows Store 8.1) versions of cocos2d-x.

libpng was updated to version 1.6.2
header files were updated for ANGLE for WP8
winrt_8.1-specific and wp_8.1-specific dirs were added for universal app
curl and websockets were updated
universal app project files added for Box2D