Don't die if metaflac is unable to extract image.
use gsed if it exists.

The standard sed that comes with OSX has unsupported switches.
attempt to use gnu-sed if it exists at gsed (as prefixed by home-brew).
use stat instead of du.

Its a cleaner way to get the file size and supported by OSX out-of-the-box.
Don't exit after converting first file.
use diff instead of md5sum.

md5sum isn't available on OSX by default.
use inbuilt output switch instead of stdout.
make sure that art files are always PNG.
clean some whitespace.
convert with afconvert if it's available.
Remove broken test.

Don't check flac -> wav -> alac -> wav produce identical waves.
It doesn't seem to work.