Basic and unfunctional cocos2d-x emscripten support.
* Only png should be supported at this point (no jpg, tiff nor webp)
* libfreetype, libpng and libz have been compiled with emscripten and included as third_party
* Dropped GLEW dependency.
* Should be compiled with https://github.com/azmeuk/emscripten/ to support GLFW
* Sample HelloCpp is not functional yet.