Merge pull request #2 from Monroe88/master

Pull commits from upstream
Add alternate 6 button mapping
Merge pull request #4 from fr500/master

Add alternate 6 button mapping
Merge https://github.com/ekeeke/Genesis-Plus-GX

Conflicts:
	libretro/libretro.c
Merge pull request #6 from Oggom/master

Pull latest changes
Merge https://github.com/ekeeke/Genesis-Plus-GX
add alternate 6 button mappings for the libretro port
- retropad to snes style layout
- retropad to madcatz fightpad/fightstick style layout