Added C++ version

Benchmarks show up to 500% speed up. The main slowdown is now a result
of PNG compression.
Added explanation for two languages to README
Switched to libpng

When I saw all the libraries linked by GraphicsMagick to the final
executable (there were 20 of them), I decided to switch to something
smaller. Since this project is called tlg2png anyway, libpng seemed to
be a reasonable choice.

Thanks to this small change, the conversion has sped up fivefold again.