Detect actual roms in mame rom dir and dynamically pull images

Detect which games are available by scanning the MAME roms dir. Check to
see if we already have a copy of the image for the game; if not, try to
pull it from mamedb.com
Fixup gitignore file to exclude more stuff we don't need

Don't include aof.png anymore and don't pay attention to a bunch of
garbage including stuff mame drops in our vc10 dir (must be because it's
the cwd).