Update m_core to the latest version from Alexey Pavlov
Update changelog and authors
Fix music not stopping properly
Remove redundant makefiles
Add new sound resources
Use SDL_mixer library for digital effects for SDL port, update makefiles and AUTHORS.txt accordingly
Update changelog and readme
Rename some functions to more appropriate names
Add COPYING.txt with GPL license
Update m_gfx_sdl.c to unify it with OpenDingux platform