Remove optional dependency on libpng and libjpeg and use stb_image instead.

[ventsin]

Currently through CMake options it's possible to build nana against libpng and/or libjpeg to support loading these formats directly in nana::image.

However, this usually entails users needing to figure out additional details, such as providing said libs to the build system.

It's entirely possible to avoid this problem by using a simple header-only library to provide image loading routines for these formats.
Another benefit is that other image formats could be supported easily as well, the full list of supported formats for stb_image is JPG, PNG, TGA, BMP, PSD, GIF, HDR, PIC.

From my perspective, there's no reason not to make this change.